There's all sorts of ways to see someone or something with Invisibility.
Getting something (such as paint) all over the invisible target
Get liquid over the invisible target, which will short the invisibility off
Looking for footprints/tracks or shadows
Infrared Xray Camera
Super Senses or Psychic Powers
Affecting the target's invisibility
Having True Sight
See-Thru Specs is a Sub Trope. [...] [tvtropes.org]