There are several different criteria:
If I can't move through the crowds of humanity on Main Street at a normally pretty neutral time (NOT after the fireworks, for example). Ditto for Epcot WS walkways, AK Africa or Asia areas, or DHS Streets of America.
If I have to wait in line more than 20 minutes for attractions that usually are near walk ons, like Spaceship Earth, TTA, or Triceratops Spin.
Not being able to get ADRs when we want at less-popular restaurants around the parks, or having to wait in line at normally slow times at the CS restaurants.
WDW is never going to be empty during non-holiday periods again. To some, having people around makes them feel, "oh, no! It is sooo crowded!" These days, more than ever, being educated, knowing when to go, and having a good touring plan is the biggest key to having a good experience.