Inspecting timing belts is fruitless. They rarely show signs of wear and will break without warning. It is best to change them on schedule or sooner. I change the ones on my cars and IS at 50% of the recommended schedule. Also, you should change the tension pulleys.
But isn't this the whole problem with the GL1500? There is no recommended replacement time/mileage, or anything, other than inspection at 100K miles.
So when do you replace them?
I just did mine for my GL1500 GW 1988 60K miles. Only did it because I didn't like their possible age. Belts where Honda's, so most likely original. Bent them backwards to inspect, and no cracking or signs of wear/damage.
Like mentioned. To inspect takes all the time, might as well just change them out at inspection time, then don't worry again.
Me, I would do it at 100K or 10 years from now on.