Found cell phone battery, how do i know its safe!

I have the Palm Pixi. I have had it for about 10 months and the battery is beginning to slowly die. I found another Palm Battery(same one as my phone) randomly but I don’t know where it came from, if it is bad, or if it is safe to put in my phone… So I have no idea how to test it out at all…
I want to know because if it does work fine, I pretty much have a second battery for my phone, which would be amazing.
Please provide as much info as possiblee. Thankyouu.