There are several ways to identify value of specific cards, but a whole collection will take a lot of time, and a lot more estimation.
Websites like
Troll and Toad will give you the most accurate pricing of any card that you might have - but you can imagine the time needed to add up your entire collection. Though, it'd be nice to type in your big shiny EXes and GXes. Remember that the site tells you the absolute value of the card given its rarity and whatnot - most cards out there will cost slightly more through shipping and well, trade.
You can also use nearby TCG stores to kind of weigh your 'bulk' if you have a lot of unneeded cards. Typically 1000 bulk cards will be valued at around 30-40 bucks, meaning that certain stores around my area will buy 1000 random Pokemon cards for that price, then sell cards in that collection for a quarter ($0.25) per card, whether or not it's holo.