This could be a good question with more details. It sounds like you want to detect at the AVR when the user presses enter at the PC? And that you are using a serial line between the two.
Please provide a copy of the C code running on the AVR. If possible distill or simplify it as much as possible to highlight this particular question.
Finally, on the AVR are you setting an LED on, after the AVR receives a CR
or CR
+LF
?