How to tell if my handlebar is really too long?
Presuming as you are talking cutting down they are a flat bar.
Slide the grips, brakes and shifter inboard and ride it, see if it feels better.  If so, cut it down, if not, leave as is. If you are still not sure,  a cheap shorter bar to try for a bit a longer might be worth the investment before committing to cutting.
Bars can always be cut down, so its cheaper for the supply chain to supply one length, that too long for most people, and have them trimmed.  Many have markings to show where to cut when trimming.