Easy way to check on how much rubber is left- is by the number of punctures you get. Thin rubber will allow the thorn- glass- piece of flint to penetrate to the tube easier and puncture it. Saying that I had a puncture first ride out on the new bike last year.

An easy way to check how much wear is left on the tyre is to look at the contact point on the road. Nice and round- like a new tyre and plenty of rubber left. As soon as it starts getting a "Flat" area across the tyre it is getting worn. Had a pair of race tyres a few years ago and after only 500 miles I started to see a flat area on the front tyre. Never seen that before. Checked the back tyre and the "Flat" are a was about 1 cm wide. Took the tyre off and the rubber was very thin. No sign of cord showing and no Punctures either.

You will always wear out the rear tyre first- so if you want- Buy one tyre only and put the Nearly new front tyre on the back and the new tyre on the front. Or just keep replacing the rear- or if you want to waste money- replace both tyres when one is worn- Like I do.