How to tell if my handlebar is really too long? As some of you already know, I'm putting together a trekking bike. It is mostly to commute where the usual route will be 15km: up to 10km on paved roads and the rest unpaved, including a highway construction site and woods. I also want to be able to take it bikepacking to Bieszczady, the Polish low mountains.
I actually put it together yesterday, and it works pretty great. The frame might've been a bit smaller, but it is quite comfortable.
My friend, who assembled like a hundred bikes so far, says my handlebar is a bit too long for me. He has seen me ride it and still thinks so. I don't  have any comparison, but for me it is really comfortable as it is, without shortening it. But cutting is irreversible, so I am reluctant.
How to tell if I really really need a shorter handlebar?

I'm 194cm tall and 115kg. My shoulders are about 55cm wide, and my handlebar with grips is 65cm wide.
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.