Battery left in storage at max capacity.
 

I sold most of my RC gear last year. I never got a battery sold for some reason and I came across it last night. It's at max capacity and it's been that way for over 8 months. Is there a way I can test this to make sure it's still usable/safe? Or should I just have it disposed?

Stats: 4s 5400 mAh

You could try cycling it. Use a light bulb if you have to and monitor the voltage with a multimeter. I'd bet it will work like new with a couple cycles on it.

I'd take a voltage reading. I charged some batteries up Saturday morning and never got to use one Saturday. When I went to discharge last night it was at 4.1v per cell, so it had already started to decline. However, when left at 3.8 per cell, it sat there for almost 2 months.

If it hasn't puffed it's probably safe to use. But it's likely lost capacity and have higher internal resistance (essentially a lower C, less punch and current/power capability).