How do i know when i need new trainers?

[buzzy1]

Probably a silly question, but I've had the same trainers for years, lost count how many.. they are still fine on the surface, no obvious signs of wear and tear, so how can I work out when they need replacing? I've almost forgotten how new ones should feel so they are probably more worn than I realise..

I was told every 500 miles. May depend on what you use them for - gym/running ... is the tread looking worn?

Thanks blue., I've used them mainly for the gym and exercise classes like circuits etc. but that includes running on the treadmill. Good point about tread, just had a look and they seem a bit worn at the heal. Think I'll just get a new pair to be sure, probably about time I did!