I am guessing in old times it wasn't easy like today to confirm the real identity of people. So my question is had there ever been a case where someone looking like old king claimed he was him and took the throne or after a king died a dude lied and claimed he was his son and took the throne.
This was what the Persian king Darius claimed about his predecessor, though this is heavily doubted by modern historians as u/Trevor_Culley (who has his 'cake-day' today, congrats!) explains here
Thanks to u/gynnis_scholasticus for sharing my older answer. That is the primary example I would have brought up, but there is at least one other example in the Behistun Inscription I want to point out. Technically, you can read the inscription as Darius saying that all of the rebels were impostors. Many of them are described as "This is [Name]. He lied saying, 'I am [alternate name], son of [other name], King of [place].'" Personally, I think the more reasonable reading is that the alternate names are just regnal names, not false identities in most cases. That said, I actually wrote about Arakha yesterday. He was an Armenian who claimed to be "Nebuchadnezzar, son of Nabonidus." It's probably the most plausible impersonation out of the whole list of supposed Liar Kings.
Several kings of so-called "Civil War" period medieval Norway (1130-1240) who had come out of Norway also claimed themselves as an illegitimate son of the deceased king.
Their claim was often backed up for the "patron" local magnate and ordeal (hot iron) was sometimes employed against either on the alleged bastard or on his mother to prove the paternity.
The most illustrative (linked to the modern illustration around 1900) case is probably that of King Harald Gille (r. 1136-39), alleged bastard of King Magnus Barefoot (r. 1093-1103) when the late king slept with the local woman during his two expeditions into the British Isles, narrated in Heimskringla:
"Jóan smjǫrbalti’s (Butter Mound’s) son Hallkell húkr (Bent) was a landed man in Moerr. He travelled west across the sea and all the way to the Suðreyjar [the Hebrides Isles]. There a man that was called Gillikristr came to see him from out in Ireland, saying he was King Magnús berfoettr’ [King Magnus Barefoot (r. 1093-1103]'s son. His mother was with him and said that his other name was Haraldr. Hallkell welcomed these people and took them with him to Norway and [he went] straight to see King Sigurðr with Haraldr and his mother. They announced their business to the king. King Sigurðr discussed this matter before the leading men, so that each could put forward his thoughts as he felt inclined, but they all bade him decide for himself. Then King Sigurðr had Haraldr summoned to him and says this to him, that he will not refuse Haraldr the opportunity to carry out an ordeal to prove his paternity on condition that he will give a guarantee that even if his paternity is proved to be what he says it is, Haraldr shall not claim the kingdom while King Sigurðr or the king’s son Magnús is alive, and these guarantees were given with oaths. King Sigurðr said that Haraldr was to tread over hot bars to prove his paternity, though this ordeal seemed rather demanding since he was now to perform the ordeal for his paternity, and not for the kingdom. He had already sworn oaths about that. But Haraldr agreed to this. He fasted in preparation for the ordeal by hot iron, and this ordeal was performed, which was the heaviest that has been performed in Norway, in which nine glowing ploughshares were put down, and Haraldr walked over them with bare feet with two bishops leading him. And three days later the results of the ordeal were examined. His feet were then unburned. After this King Sigurðr welcomed his kinship with araldr, but his son Magnús grew to hate Haraldr heartily, and many leading men sided with him. King Sigurðr was so confident of his popularity with all the people in the country that he asked that everyone should swear oaths that King Sigurðr’s son Magnús should be king after him, and he received oaths from all the people in the country (Magnússona saga, Chap. 26. English translation is taken from: [Finlay & Faulkes trans. 2015: 162f.]).
This Harald Gille/ Gilchrist was in turn assassinated in 1139 by another alleged bastard son of Late King Magnus, Sigurd slembidjákn ("rogue priest"). A praising poem dedicated to this Sigurd also mention that Sigurd also got ordeal to prove his paternity:
"Five bishops, who were deemed most distinguished, conducted the ordeal concerning the lord’s kinship. Proofs were given that the generous Magnús [Barefoot] was the father of that mighty king (Ívarr Ingimundarson, Sigurðarbálkr, St. 10. Link is the official site of the Skaldic Poetry project that English translation is taken)."
It is also worth remarking that there had been only three or four bishops in the early 12th century Norway and the ordeal probably took place out of Norway (according to all the later sagas that cite this stanza of the poem). This Sigurd was not recognized at least as a (co-)king of Norway, and expelled from the kingdom at first, then captured in the battle and tortured to death by the supporters of the child king Inge [himself was a bastard of assassinated King Harald].
Two more alleged bastards of assassinated King Harald Gille claimed to the throne of Norway (one was said to have been an alleged child when Harald had not visited Norway), and accepted as co-rulers of Norway together with aforesaid Inge......
We don't confirm their paternity absolutely at least based on the extant evidence for sure, but they seemed to be accepted as a late king's bastard without much dispute in these cases.
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On the other hand, we have one more much dubious but successful case of royal imposter in medieval Norwegian history - no other than King Sverre (r. 1177/1184-1202), founder of the Sverrir dynasty (1177-1319)!
Pope Innocent III (r. 1198-1216) called him as a "regecide" and "royal imposter" in his correspondence (that banned Sverre).
Sverre had originally been a lesser clergy in the Faeroe Islands and then took a visit in Norway and claimed to be a bastard son of late King Sigurd Mun (r. 1136-55), then bastard son of King Harald Gille (one of two illegitimate sons who came to Norway after Harald's assassination - see above), but probably first after his taking-over of the leadership of the rebellion faction, the Birkebeiner. In Sverris saga (officially supervised by him), the vision and the word of Prophet Samuel are employed to strengthen his and his mother's claim that Sverre's mother moved to the Faeroe Islands and Sverre was in fact a child of the late king.
What works against Sverre's claim, however, is that the extant fragmentary evidence when modern historians scrutiny his claim - Sverre was probably too old to be an illegitimate child of King Sigurd Mun (d. 1155 and probably born around 1135).
Sverre had been deacon (lesser clergy) before his visit to Norway, but the Canon Law stipulated a minimum age for the ordination to clergy - less than 25 (if we suppose that Sverre was a product of the extramarital affair of Sigurd when the later had been 14/15 years old) was probably too young as a clergy even in accordance with possibly a bit lenient standard of the fringe of Latin Christendom.
Another source, a entry of the Icelandic annals make a note that Sverre took a visit in the Orkney Isles [in the northern Isles of the British Isles] as early as in 1169 - this also suggests that Sverre had already been born around 1150 when his alleged father Sigurd was too young to be a parent.
So, though very disputed, some (or rather the majority?) of the historians have some doubts of his actual paternity.
Concerning some more historical background of Sverre as well as relevant literature, please also refer to the following previous posts of mine:
