Red Eagle grips are easy to tell if they are like the ones that Rugerguy shows above, the ones with aluminum escutcheons around the screw holes. Only the type 1, 1A and type 2 Red Eagles had this grip, up to around serial number 3700. As usual some few lower numbers won't have the aluminum escutcheons around the screw holes. Just normal overlap, as most types have. Now for the rest of the Red Eagles, it just like the Ruger Factory Stags. You can be 100% sure if they are not right, such as wrong size med., with the fat neck Eagle painted red, or the wrong color red in the Eagle. Or the wrong grips, as they should be the solid hard rubber grips not the later hollow grips.
Now if someone get the Black Eagle grips that are made just after the Red Eagles stopped being made, and gets the right color red to paint over the Eagle after removing the black, then it's going to be hard to tell that they are fake. It's just like the Factory Stags. You can tell the ones that are done wrong but you can only guess it it is done right.