Problem is Mark there is no way of knowing, without actually measuring it which is involved, requires some nasty chemicals, and some expensive equipment.
Console yourself with the fact that until 'recently' brewers didn't know the alphas of their hops either and still made good beer.
Assuming they are the same as commercial is a safe bet, and if the beer turns out under or over hopped adjust the bitterness next time. . . . I would actually use something like the 10 year average alpha for the variety you are growing, which should be a better value. . . .could ask somewhere like murphy's, simply hops or farams for that information.