In a rear derailleur the axis of the main pivot joint and upper and lower derailleur wheels should be parallel all times. Derailleur cages often get bent at crashes so that particularly parallelism of upper and lower derailleur wheels are lost leading to such as twisting of chain, etc. (Modern derailleur cages are often very thin and not of steel anymore as was in the 7speed ages). Due to the short distances involved judging by eye sight is not very reliable. Are there any special tricks or gadgets (may be similar as for derailleur hanger)?

Twist it with your hands. If it is cheap enough, replace it. Go find a donor bike at a thrift store, pawn shop, flea market, or yeard sale if money is an issue.

Bike shops have a tool that bolts on to the hanger and then rotates around to touch various points on the rim. If it touches all around the hanger is straight. The hanger is straightened by using the tool as a lever.

the question was not about a bent derailleur hanger - but about an in itself bent rear derailleur

Well If you can thread the upper bolt into something really solid,
maybe one of those bolt on Derailleur hangers clamped in a bench vise,
solidly.. then you can give it a go at bending it back.

I don't think that you can really expect it'll bend back exactly the way it bent "forward"?
Even though it might "look" straight, is it really???