To properly protect your child, the car seat must be in good shape with all labels and instructions present and readable. Dorel car seats typically have the expiration dates molded into plastic of the shell on the back of the seat or bottom of the base. You will find a statement similar to "Do Not Use This Restraint After" with the expiration for your seat. Newer models (End of Year 2016 - present) also have the expiration date noted in the instruction manual.
Do not buy a second-hand car seat.