Is my plant a monstera or philodendron?

The most important one is that Monsteras are fenestrated while split-leaf Philodendrons are not. Fenestration refers to the holes in mature Monstera leaves, and while a split-leaf Philodendron has, well, split-leaves, they lack fenestrations. Although many Philodendrons climb, true split-leaf Philodendrons do not.