Two creatures can see each other if they have line of sight to each other. To determine line of sight, draw an imaginary line between your space and the target's space. If any such line is clear (not blocked), then you have line of sight to the creature (and it has line of sight to you). The line is clear if it doesn't intersect or even touch squares that block line of sight. If you can't see the target (for instance, if you're blind or the target is invisible), you can't have line of sight to it even if you could draw an unblocked line between your space and the target's.