Would managers tell you if you are being written up? Or would I find out when the time comes to fire me?

I work at the Target Guest Service Desk, and whenever I close, I keep working a bit over time (past my schedule shift like 15-30 mins) not because I'm intentionally doing so, but I guess I get kinda behind on the work and get the job done a little late. I've been working there for two months now, and I keep getting the feeling that my managers might want to fire me soon because of my slow performance that seems to be hardly improving even after working there for that long..