Do you mean quality of older tires, or how to tell the difference between new tires?
You can pick up a tread depth gauge at an auto part store for a buck or two. Easy to use and effective. They measure in "32nds". A new tire is on average about 10/32, more if its a truck tire. 2/32 or 3/32 is the point to replace, and some states require it for inspections.
Also check for cracking and dry rot.
When buying new tires determine the following:
1. How many miles per year do you drive?
2. How long will you keep your car.
3. What weather conditions do you encounter during the year?
4. Do you swap out snow tires in the winter or run an all seasonal tire year round?