I use a fairly simple method. I ask real pilots who are familiar with both the real world aircraft in question and its counterpart(s) available for the sim.
There's a flight sim news group I frequent where a few real pilots also participate (alt.games.microsoft.flight-sim). One of these pilots is Dudley Henriques, who has extensive real world experience with over 70 different aircraft...both GA and military. He's retired from flying and instructing now (primarily as a fighter pilot and aerobatic performer/instructor), and now enjoys "flying" with fs2004. He does offer his expertise to some sim plane developers as well. Dudley is very generous with his expertise, and is happy to help us simulated pilots with all sorts of issues...including letting us know if a certain sim plane has a fairly accurate flight model. His endorsement of the RealAir Super Decathlon was one reason I purchased that plane. For those who like fast military type planes, he also recommends the MIG 21 from Captain Sim.
Another now retired yet very experienced pilot I depend on is Peter McLeland. Peter's endorsement of the DreamFleet Piper Archer II helped me decide upon that particular acquisition. There are a few other pilots here and there who have commented on the relative realism of various simulated planes, and I'm always very interested in what they have to say.
So...since I don't
yet fly real planes myself, I feel the best I can do to find planes for the sim that are "as real as it gets" (within obvious limits, of course), is to ask people who have flown the real planes I'm interested in before I decide to add them to my sim.
Melissa