Use a RF receiver with TTL level outputs, and monitor those on the PC. If the PC has a parallel port, then maybe you can use that.
Needs to be USB? Then try using a USB-UART converter (TTL level not RS232/DB-9) that has handshaking lines (DTR/DSR/RTS/CTS). You can then use the DCD, RI, DSR and CTS handshaking lines as digital inputs. You can open the (Virtual) COM port using whatever software you are using (even a scripting language) and monitor the status of the handshaking lines either through polling or events.
If I was searching eBay for suitable USB-UART modules, I'd try using "USB (UART,serial,RS232,RS-232) ftdi". You will find plenty of modules looking exactly like
this one. Note the connections for the modem control (handshake) signals, that is the important part. Pick your favorite far east seller, wait 2 to 4 weeks, and start playing. If you want joy sooner, you can no doubt get identical modules from any domestic place hocking arduino-type goodies at several times the price, plus shipping.