How much do the Russians know about USAF aircraft?

Americans seem to know everything about Russian aircraft like Mig29 and Su27 variants. The Americans regularly practice against these aircraft and i believe they bought some Mig29s from one of the former Soviet states. Not too many countries are in a position to refuse any American requests to examine their Russian built aircraft.

Do the Russians go out of their way to closely examine US built aircraft like the latest variants of F15, F16 and F18 etc.?

I would imagine they learn things through espionage but are there any examples of a country allowing the Russians to closely examine and assess an American built aircraft? The latest US radar and avionics would probably be of greatest interest to the Russians.