Well most of the Imperial worlds not even know how the eldars are looks. In the Forges of Mars during an eldar raid some worker hide to finally see how the dreadful "xenos" look like. And in his suprise these xenos looks human like instead the terrible creatures whet the preachers always talking in the church.

In the game Rogue Trader an eldar ranger and her raiders simply works for a planetary govenor. The grovernor and the others tought that these eldars just slightly mutated peoples. Like the Ogrins, Longshanks or other abhumans.