There obviously won't be a single answer that fits all circumstances, but here is my pennyworth.
If a result is sufficiently accepted by experts you have good reason to trust, then the result can be trusted. (Obviously the better you understand it the better, but sometimes one has to save time.)
If a result does not satisfy the first criterion, then be very suspicious of it unless you are given, or can think of, some accompanying reason for its being true (rather than a long calculation that just happens to work).
