How to tell if my handlebar is really too long?

Some thoughts:

Comfort is certainly a factor, and if that's the primary consideration, it sounds like you're good to go. A bike that will be ridden on a nice paved trail should be primarily comfortable. If that's not the use case, read on.

Does the handlebar width allow you to get good arm and hand posture? That is, when you're in a high-force situation (say, going down a rocky incline where some jarring has to be managed), are you able to have a straight force line from your brake levers through your wrist and up your arms, or is your wrist form an angle with your forearms? Similarly, is the top of your wrist reasonably flat with your forearm, or is the bottom of your wrist twisted down? A summary-- are you having to choose between a good grip and being able to use your brakes?

How efficiently are you riding? Obviously, the importance of this one varies according to your discipline-- as you move from XC toward DH, control becomes more prized over efficiency. Width that causes you to have to constantly tension your shoulders to put power down efficiently is not good for XC, but might be more acceptable for enduro/DH, especially as it means greater stability and control.

Is there any chance your builder friend might have a couple of narrower bars you could try out? That might give you some peace of mind if you do find yourself about to saw some length off your bar.

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.