Depending on the manufacturer and age of the seat, it could be listed on the seat someplace. Each manufacturer is a little different. Some say 6 years, some say 7-8....the absolute do not use after time for any manufacturer is 10 years. That is a pretty long time, though, and it makes sense to find out what each manufacturer says about the actual seats you have.
If you can't find a date stamped into the plastic of the seat or near the serial number sticker, call the manufacturer's 800 number to find out for sure. If you surf the manufacturer's website, there may be some information there as well.