Is cool.
You can tell when your tyre is worn worn when, in no particular order:
It is worn down to or well into its wear bars. Mine regularly go into the bars.... but that does no make them legal or unworn as if by magic. I just go on what it looks like, how it feels and if it is likely to last my journey. If it is shot, or will not last until I can sensibly get a replacement, I get a new tyre fitted. That may be say 500 miles 'early' if I plan to be out'n'about in Europe.... but I do not mind getting them done abroad either..... the money lost in the 500 miles is peanuts over the bike's (or my) lifetime... I trade it off against the worn tyres I have over run.
It's tread depth is less than the legal minimum. Google for info.
The bike's handling goes to pot. Tracking groves in the road, squirming excessively on over-banding or on white road markings. You know what it is; so if you can live with it your worn tyres will last longer than someone who thinks they will die at every shimmy.
It does not fall / roll into corners properly. Look at a new tyre. It has a nice rounded shape. As the tyre wears, more often than not it squares off, losing its nice round curve, replaced by a sort of flat plateau, or even a cup shaped 'dent' around the tyre's innermost diameter. As you lean the bike over, the bike has to ride up and over the rubber between the flat plateau and the still rounded edges.
You can see cord / canvas / wire showing through the carcass.
You get an MOT fail / rectification notice / endorsements / fine from plod.
A mechanic tells you.
Front tyre wear is different to rear, not least as the size and external pressures on a front tyre are different to the rear's. Mine often feather / flat spot on the outer edges. You can feel it starting to happen.
Wear does not occur overnight. So remember what your new tyres feel like and when you feel that feeling has gone, swap 'em for new ones, or at least check your current tyres for condition and pressure (which I am sure you do anyway).
Worry less (or not at all) about the mileage of other bods. A fellow commuting 300 miles a day on a motorway may well hack through (badly square off) tyres way quicker than bod XYZ who just tootles to Tesco for root vegetables every third month, whose tyres will probably last half a life time. A bod hooning around the mountains will be somewhere in between. Some bods report being reluctant to go above 5,000 RPM.... theirs probably never wear out at all.
If your tyres are are worn out, bin 'em, irrespective of mileage. It really is that simple.
but how do you know when its worn out?).
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