You can assess AFTER replacing whether the belt was still healthy when it was changed. You do this by turning the belt inside-out and inspecting closely with good lighting and perhaps magnification, for minute cracking, particularly at the tooth roots. (Of course, you should never do this, or even bend the correct way, smaller than the crank pulley diameter, any belt you plan to use/re-use.)
Again, I'll post pics after removal, inside-out per the above.
For what it's worth, every belt I've ever done, even way overdue, has come off looking very good, with no notable cracking. This in no way means that I advocate being remiss, however!!
Interestingly, I had a couple of old Subaru's, years ago and they had inspection ports to look at the belt, but you could only see the drivers-side camshaft pulley and maybe 4 inches on either side, so it's benefit was questionable. Nevertheless, you could see the gross condition (not frayed, no chunks missing, etc.) and so it was SOMEWHAT helpful. If you've never changed a Subaru timing belt, you haven't lived. Some have 4 cams and they have a tensioner, it goes around the water pump and then there are TWO idler pulleys. You have to be very creative to hold everything in place while you put the belt on, for sure - lol. In any event, ALL Subaru's leak oil freakin everywhere and so there's always oil inside the belt cover and yet I've not seen one even close to failure...
Anyway - too much caffeine today and WAY too many words - lol