They Start Texting You About Things That Have Nothing to Do With Hooking Up
The biggest tell is usually the simplest one. When your friends with benefits starts sending you messages about their day, sharing funny videos, asking how your job interview went, or texting you good morning without any mention of meeting up, the dynamic has shifted. A purely physical arrangement revolves around logistics - when, where, what time. When someone starts treating you like a confidant rather than a hookup, feelings are almost certainly involved.
Pay attention to the tone of messages too...