It's the same problem with autovectorization. Often the only way to tell whether a compiler optimization is successful is to check the produced assembly or benchmark the code.
It seems that for devirtualization GCC has a warning option -Wsuggest-final-types which is supposed to tell when devirtualization fails in link-time optimization. Not sure how reliable that is, or whether it will produce gobs of unhelpful warnings. Maybe it could be combined with some kind of hint that we want this particular call to be devirtualized, and don't care about calls without the hint.
It seems that for devirtualization GCC has a warning option -Wsuggest-final-types which is supposed to tell when devirtualization fails in link-time optimization. Not sure how reliable that is, or whether it will produce gobs of unhelpful warnings. Maybe it could be combined with some kind of hint that we want this particular call to be devirtualized, and don't care about calls without the hint.