Tires: Tires on a Used Car. Are they good?, 2003 lexus ls430, goodyear assurance tires

Question
In the past year I purchased two used cars

1995 Mercedes E420 with four matched Goodyear Assurance  tires - with "enough tread that I can grab it" (plenty of tread).  [unadirectional tires]

2003 Lexus LS430 with four matched Goodyear Eagle LS tires.  With good tread, but not as much as the other car.

What else do I look for to determine the tires are safe to drive?  Do tires dry rot?    With the Mercedes, I've noticed the tires seem to squeak occasionally.  All tires on all cars have the manufacturer's recommended inflation.  I'm thinking the Mercedes tires, that squeak, don't hold the road as well.

Summary:
Other than tread depth, how do you determine if tires on a used car (that is - no history) need replaced?

Answer
Bill,

 The first thing to look at would be the DOT, this is a 10 -12 digit letter, number or a combination letter/number generally located near the bead of the tire.  Sometimes on both sides but, it can be only on one side.  The last 3 - 4 digits are the week and year of manufacture...the last 2 being the year. If the tires are older than 4 years I would replace them.  Tires do not "Dry Rot" they do however develop a condition known as Weather Checking or Ozone cracking.  Look closely around the bead area and in the grooves of the tread.  If you see any it is also a good idea to replace them.

 As for a "squeak" if it is indeed a squeak and not a squeal then it is time to have the ball joints checked.