Welcome littleRhody
I too was a lurker back when I had a life
look at me know
1) HOW DO YOU KNOW WHEN YOUR BOW NEEDS TO BE TUNED?
A better question is how do you know when it IS in tune. A bow always need to be in tune. and there are many diff. methods to do it. I would suggest visiting the EastonArchery.com site and downloading there tuning guide. Lots of great info.
2) HOW DO YOU TUNE IT YOURSELF?
See above
3) HOW MUCH WOULD THE LOCAL SPORT STORE CHARGE?
I don' t know 20 to 30 bucks maybe, but YOU have to be the one shooting it and they should help you get it in tune for your style.
Good Luck