When mono has been compromised, you can tell by feeling it and looking at it. It will feel and behave differently than the good stuff. As many have said, without having been exposed to the elements and/or foreign substances, mono will last (without compromise) for many years.

I think my oldest bulk spool of 15lb UG is going on 5 years old and it still knots like it is brand new and breaks under about 35lbs of static load