1) Things start drastically changing around you - for example, your boss tells you that you need to compile all of your data pertaining to your recent project and present it to everyone in the lab so we can discuss. WHAT?!?! A lab meeting?? We don't have those! Except, we are. On Monday. Over coffee or lunch - what?!? Yep, there it is. That's more like it.
2) You answer the door to your lab and the prof that's at the door takes a look at you and says "Oh hey! You're still here?"
3) Your boss comes in and tells you he's sending you to Uganda for 3-6 months, once you're done with your thesis.