You don't unless you ask them specifically. Tinseth seems to be the most "popular" from what I read. Best you can do is pick one and get familiar with the extraction efficiency of the method. Turns out that when compared to actual GC tests, none of the methods are exact. Too many variables at play here.
I just started using Beersmith and I like it a lot, but I had to switch the bittering calculator from Tinseth to Rager in order to get anywhere near the IBU estimates based on BJCP style types. Tinseth seems to give me REALLY low IBU estimates, even with a 4 gallon boil for extract or PM brews (unfortunately I'm not yet equipped for AG yet).