First use a straight edge across the front edge to see if the
pulleys look
right.

Won't work if the outside edges are different castings or pressings.
Better is to drop a dowel with a diameter of the belt width so it
doesn't touch the bottom of the groove. The dowel can be just barely
the length of a suspended leg of the belt so it's easy to get in
there. Take a look at how the dowel rests in the groove, using ink to
help if it's tough to get a lotta light and a dental mirror in there.
Do it at several positions of the pulleys. If it's not right on the
money you've got a problem.

What confuses me is I can't locate a carbon fiber/kevlar(?) toothed
belt (they run cooler) for my motor. It's a very short belt, from a 4"
to a 2.2" pulley. The ones I use are stretchy and require adjustment
during the belt's service life. I'd pay for one that didn't. I think
they're for lawnmovers or something. Is stretch a design consideration
in alternator belts? Sort of a high load slipping clutch like the
doughnuts in an outboard propellor?