It's not exactly the same time period, but I know that some of the sources for gestures in Medieval Iceland are textual. I think this is related enough to be an appropriate response (since it's about knowing what gestures were like). I'm not familiar with drawings or depictions of Northern European gestures, so I can't tell you anything about that unfortunately.

Basically, in some of the Icelandic sagas, gestures are described (although this can be extremely short). This also counts for obscene gestures, although they are very scarce (as the citation below demonstrates). The person I'm quoting wrote an entire article on all the kinds of gestures found in the Icelandic Sagas (and other tales), but I don't have the time to summarize the entire thing here.

"Gestures signifying dislike, contempt, and scorn are uncommon in the Sagas and Tales of Icelanders, and there are, for instance, no examples of spitting, sticking the tongue out, or baring the buttocks.There is one example in Grettis saga of giving someone the finger. The episode takes place in a church in Norway, where Grettir was to carry hot iron in order to prove his innocence in the death of the sons of Þórir and their companions. As Grettir walked down the aisle, a young boy ran up, and, in a tirade of words, accused him of being a criminal. The boy also gave Grettir the finger (“rétti honum fingr” [1:1016.24]), made faces at him, and called him names. Grettir killed him on the spot, and the saga comments that since no one knew where he came from or what became of him, it was generally believed that he was an evil spirit."

So, we know that giving someone the finger was actually a thing in Medieval Icelandic society, but that it wasn't particularly appreciated. It's kind of important to remember that the sagas tend to have some exaggeration, so it's possible the finger didn't always get you killed.

"Similarly, threatening gestures and physical expressions of anxiety or discomfort are rare in the texts examined and limited to incidents in Ljósvetninga saga and Eyrbyggja saga."

Basically, a dude slammed his fist on a table to demonstrate his strength. It isn't really an obscene gesture, but it comes close in my opinion (also given the context).

Lastly, "Gestures accompanying or showing feelings of sadness or grief are non-existent, the only exception being Vápnfirðinga saga[...]"

I'm only putting this here to show you to take these things with a grain of salt. It's very likely people cried and grieved a lot. Sagas can be informative, but they aren't always reliable sources. The scarcity of descriptions of these kinds of gestures doesn't mean they didn't exist.

From: Wolf, Kirsten. 2013. Body Language in Medieval Iceland: Gesticulation in the Sagas and Tales of Icelanders. Scripta Islandica 64: 99–121.

I hope this is at least answers your question partially.

You might refer to Elworthy's The Evil Eye -- some obscene gestures were transferred to necklace-hung pieces and the like to more consistently distract the evil eye, and I believe he also discussed graffiti and the like, if memory serves.

Unfortunately, I don't have an exhaustive knowledge of all obscene gestures that the above cultures used, but I do have extensive knowledge of one in particular: the ficus manus of ancient Rome. So, perhaps, I can answer part of the how we know.

The ficus manus was a closed fist with the thumb tucked between the index and middle finger - or sometimes portrayed in different positions. This was carved into or carved on a number of amulets which would have been worn as a kind of prophylactic against the evil eye. Expressing obscenity for apotropaic (lit: to turn away) protections is rather common throughout the world as well as the cultures above in the ancient Mediterranean. Heres a NSFW (well, kind of. Better safe than sorry) picture of an apotropaic amulet with the ficus manus on the left, phallus to the right, and male genitals below.

The ficus manus (lit: fig hand) itself represented the external female genetalia which was sometimes referred to in Classical Latin as a ficus or fig. For more information on apotropaic uses of sexual symbols you might want to consider digging through J.N. Adams The Latin Sexual Vocabulary. He offers an explanation of both apotropaic obscenity and why certain symbols such as the fig are obscene. Although he primarily focuses on the linguistics of sexual obscenity, some of it does bleed over to symbols such as the fig.

Finally, while I don't know specifically of any examples, obscene gestures may have been preserved in some frescos - likely Pompeii - perhaps in the context of a comedy or farce scene. Or preserved in satire such as the satyricon.