How to assess tyre wear/fresh tyres

[Loheswaran]

I do 1/12th - and TC - though I have had a 1yr lay off.

Now I have a really stupid question, so please don't laugh at me. How do off road racers asses tyre wear - is it to do with how far the spikes wear down, or is it to do with the fact that they dry out?

I only ask this coz with TC the tyres come off the rims, and they're just rubbish. with 1/12th - its a combination of size change, chunking, coming off the rim, and drying out.