Long story short...

I found some 20# P line mono that is still on the original spool that it comes on when you buy it at the store. It was in a bag (vinyl bag if that matters) that has been sitting in the garage and shed for 5-7 years now. Weather here in the summer gets 100-120°F.

Question: How do I know if this line is still good to use?

Just checked at this moment and line hasn't been used yet (still 300 yards on it.) Looks to be in great condition.

I wouldn't, not if it's been in that kind of heat for several years now. You can get Pline for less than $10 for filler spool, I wouldn't take the chance of losing an expensive bait or big fish. 

Thanks. I just tested the line with an over hand knot. Broke the line using my hands. Out the trash it goes.