Three pronged argument of the credentials of a player are production, film/ playing style and measurables. Other things to consider are intelligence, work ethic, coachability, intangibles etc.
For measurables' evaluations: figure out the "eye test" measurables the NFL looks for at RB. If your tailback doesn't run sub 4.65 than it can send up a red flag although there are exceptions like, Ryan Torain, Arian Foster (they time almost exactly the eye test time) and LeGarrette Blount is a little slower at 4.7. Mike Hart who was moderately successful for the Colts last year ran a 4.69 at the NFL combine at only 5'9 about 200 lbs. Ahmad Bradshaw runs a 4.6 flat.
If you want a good glimpse of power look at highlights of Peyton Hillis (speaking of Hillis- he ran a 4.58) Owen Schmidt, Michael Bush or Brandon Jacobs. A good glimpse of freakish agility is of course Barry Sanders, Marshall Faulk, Danny Woodhead, Justin Beaver etc. or you could look at a high school highlight tape such as Nile Knapp, Noel Devine, Eric Gerber or John Howell.