German claims were more scrutinised than those of other nations. To gain an award a pilot needed written corroboration from another aerial observer, and preferably written confirmation also from a ground observer (or wreckage). Your information is not correct about the B-17s: during 1943 the Germans introduced a credit system for four-engined bombers where a pilot would gain credit for a 'separation', that is, damaging a bomber so it had to leave its formation (and thus become far more vulnerable). But there system was applied rigidly and, being over home territory, claims were more easily verified.

Recent historical analysis has compared the returns of each side and shed light on over-claiming, but on the Eastern Front this is often less straightforward. But there are a large number of factors that make the massive claims on the Eastern Front, compared to the West, very credible. These include: Luftwaffe pilots flew almost continuously, their 'tours' did not end and many fought for years on end; they flew multiple sorties per day from airfields near the front line; outnumbered, more of their sorties resulted in 'contact'; it took the Soviets a long time to have the time and resources to train pilots properly. The Luftwaffe also encouraged a 'star' system, with the lion's share of the kills going to relatively few pilots.

However, a pilot making routine false claims would have to suffer the suspicion of his fellow pilots, rival units and his intelligence officer, who was more concerned with calculating enemy strength than making a good story for the people at home. Many Allied pilots made most of their claims in short periods of intense combat (such as the Battle of Britain or over Malta). When once considers the period of front line service for men like Hartmann, Rudel etc, and the relative quality of the opposition for most of the period, the numbers are very credible.

I am reasonably familiar with aerial claims in the Pacific Theater, which were grossly inaccurate. On August 24 1942 the Japanese sent 41 aircraft to bomb Henderson Field, 20 (or 23) Bettys and the rest Zeros and Kates. The defenders claimed 28 while actual losses were four single-engined aircraft. Japanese claims were similarly optimistic. The raids over Rabaul saw similar over-claiming by US P-38 and Corsair units: one day in January 1944 saw US aircrew gain 27 credited victories, for only three Zeros shot down (the Japanese claimed 34 fighters and 13 bombers, one bomber being shot down and 7 fighters to all causes). In Burma Japanese claims, mainly by 64th and 50th Sentai, were usually at least 50% higher than RAF losses.

The point, however, is that in almost all cases these claims were made in good faith. Aerial combat is of course hugely confusing, most aircraft of the day would trail smoke under certain conditions (when g-forces, velocity, throttle control and engine fuelling systems made engines 'cough' and smoke), and of course many pilots might 'have a squirt' at a stricken enemy.

I'm aware of two cases of seemingly deliberate, dishonest over-claiming: Satoru Anabuki's claims of October 8, 1943, and those of Robert Hanson of VMF-215 over Rabaul in January 1944.

Figures in Europe seem to be far more robust for the most part, comparing for example the Battle of Britain and various Allied bomber raids where data for both sides is fairly clear, and where far fewer lost aircraft would 'disappear' into ocean or jungle.

Useful sources: Duel for the Sky Christopher Shores (anything by Shores is good, he has books on the Asia and Malta fighting, too, with exhaustive data research), Target: Rabaul Bruce Gamble, esp p300-310; Bf109 Aces of the Russian Front John Weal.