there are tricks for that (to identify gperm if theres no hints (no blocks/headlights = Ra, Rb, F, Ga, Gc (one of those)
i know some top cubers can do it (though they dont do it for every case i believe)
rowan for example i heard can do it
another question, what do you guys think about R-OLL? worth learning? seems it would be useful in predicting PLL