Hey Snooker,
I mainly use eBay completed listings and cardtarget. VCP is another source that quite a few people use. Once you become more involved and consistent with buying whatever it is you collect, it will not take long before you can use your experience as a guide too.
The hardest prices to gauge are the tough, rare, and scarce type of cards because they may not come around too often to get an accurate price on their past sales. I also use friends to lean on if I need help....that's always a good source to put in your back pocket!
Good luck!