By the amount of flow thru the heads at a given vacuum.
What the flow numbers will say is that at given flow (and they head supplier should be able to give numbers from .1" up in .05" increments) there is x flow in cfm (corrected to 28" h2o and temps etc) which equates to a potential HP figure.
Thats the best you can do with head numbers alone. You need to know other stuff to polish the numbers.
Here is a post I started a while back. http://www.fordmuscle.com/phpBB/viewtopic.php?topic=4116&forum=1
From what I have found the general rule is flow @ 28" h2o x .257 x number of cyls = HP
Some other sites with calculators end up with results that validate this formula.