I know of only one case where there is an unpainted Rocketfett that has the lettering on the legs. Evidence of copyright removal is definitely a sign of forgery. As always, compare what you see with published photos. This is your number one way to make sure that what you are buying is authentic. Most of the fake ones thus far are made to resemble the unpainted version, though the similarities of the J-slot type to the production figures may change that in the future.
Common sense plays an important role as well.
If you can't determine how your seller ever got the figure in the first place then just walk away. Do not let your desire to find this figure outweigh your own reasoning skills. Always approach a Rocketfett from the aspect that it is fake and then prove that it meets all the criteria of being legitimate before you get excited about it. Remember that these things are extremely rare. There may be 100 total in existence, though only around 30 are accounted for and known to be in collector's hands right now.