Calculating value before the flop

[MortimerJazz]

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,

[spoonitnow]

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