There are some posts out there on the net about how to measure the alpha acid content by titration. You boil up some hops, add an indicator (phenythalien -maybe not spelled right) and then add a sodium hydroxide solution until a slight red color develops. Then there is an equation that you plug the ammount of hydroxide into that gives you your AA content. I tried it once when I fist started growing hops and found it to be way too much of a PITA. I would just go off the smell and wing it. I grow cascade, centennial and CTZ. The cascade is a lower alpha hop and has a relatively mild odor compared to my other two varieties. The CTZ is a very high alpha hop and the cones are very smelly and pungent. After sqeezing a few of them, my fingers are all sticky. So, just do your best guess and then brew up a batch of medium to low IBU beer and make adjustments from there. Or you could just use them for finishing/aroma hops or for dry hopping.