Look for explicit statement in job offer or just send question before applying, I did that many times and usually got some response. If you don't get any response then it means that it was not worth applying anyway. :)
"Java or C++" is such type of key phrase/statement, but there are other ones, depends on job profile you are looking for. Front end devs should rather avoid job offers, which mention some particular javascript library as an must have requirement.
Question is simple: I don't have a lot of professional experience in required language/technology, but (here goes something interesting about you), should I apply to your job offer?