AT150MLX replacement stylus

The 125 will fit, but that stylus was discontinued a couple of decades ago or more. I'd be concerned that the suspension has dried out and hardened, which is always a potential issue with vintage and NOS styli. If you have one already, it won't hurt to try it, but I would not recommend buying one.

I don't have a suitable microscope either, but have seen microscope photos from reliable sources. It can be tricky to get the lighting right and you need a 200X+ magnification. The cheap USB things don't really work either.

In one case I sent a cartridge out to a retipping specialist to get inspected and they sent me back photos and test results. I don't know if that is an option in your area.

The easiest thing for most people to do is just count sides and then divide by 3 to track hours. I use a phone app but you can also get a cheap clicker counter.

With these ML styli, what happens is the "fin" at the peak gets worn down over time. When the peak is fully worn down the contact points on the tip widen and that can cause the stylus to almost hit the bottom of the groove which results in excess noise. In some cases with severe wear the stylus can turn into a sharp cutting tool which is not good for your records.