I check mine every 20,000 km and replace it at each 90,000 km.
Mine is an easy job, because I have the forks out for the service so I have this view.

You won’t be in as far as this because that’s a cam chain replacement, but you get the idea. The front will be off when it gets home to fit the timing gears, so it will get it’s (slightly overdue) 3rd belt then.
I have always just used the method of twisting the belt 90 deg in situ. to check the adjustment. If you can comfortably do that with your fingers, I call it good.
A long Allen key is useful to reach the adjuster and I think the lock nut is 12 or 13 mm.
It seems easier with the tank off too.
Can’t think of anything else…
