I have been told and i am begining to belive it as i have been looking at every clown i have come across the easiest way to tell is this.
Look at their eyes. My false percs have orange rings around their Pupil. All of the true percs i have seen (or ones labled true percs in the LFS) do not have this their eyes are solid black.
Feel free to chime in if i am wrong this is based purely on my observation.