Calculating value before the flop

I've been reading Harrington on Hold 'Em and in one of the questions that come at the end of each chapter, he puts the reader in a position of have 6c 8c and with the opportunity to call $100 into a $650 pot.

He calls the decision "an easy call" based on the flush and straight possibilities.

Now I can work out that the pot-odds you're getting are obviously 6.5-1 ... but what I don't understand is how I can work out anything to compare them to? The rule of 4 and 2 give me an idea of my chances of improving my hand on the turn and the river (which I can then compare to the pot odds I'm getting), but how can I figure out my odds of getting a straight or a flush before the flop has even landed?

Thanks alot for your help,

Check out http://www.flopturnriver.com/Common-Flop-Odds.html for a list of a lot of fairly common odds on flopping different things.