I was in Malcolm's for my tracking done a few years ago and while I was there someone came in and asked them to take a look at one of his tyres that they (Malcolm's) had only fitted on his car a few days ago because something didn't feel right on one tyre. He was very polite and just asking for it looking at. Malcolm said not a problem and went and checked. He only looked at the one tyre he'd been asked to but two minutes later he went back over to the customer and told him he wasn't happy with that tyre so if the customer didn't mind he'd like to take all four back off and put another brand new set on for piece of mind. Excellent service and one very happy customer
Couple of months ago thought I needed four new tyres - Malcolm had a look - 2 needed replacing but the other two would 'be o.k. for another 1000/2000 miles.' -- I can play the piano but what goes on beneath the bonnet is a complete mystery to me (think I know where the oil goes!) and so we value 'honest' car mechanics.
You can check tyre tread with the '20p' test. If you put it into the groove, you shouldn't be able to see the outer band of the coin. If you can, your tyre is below the legal limit.