You can get an RS232 Tester from Radio Shack #276-1401 for $15. Put it on the end of the RS232 cable next to the CNC without connecting it to the CNC. You should see RD lit, then flicker once you transmit. This will prove data is getting to the end of the cable. However, depending on the software and wiring, your cable will need to built correctly. If you are using software flow control, most of do, only 2, 3 and 7 are important. You must jump 6-8-20 together at the CNC. The PC does not matter.
If you have a bad soldier job or the pins are loose, you will see this! I have! Check the connector.
Also, you can eliminiate that game by using a hardware flow control cable, crossing 4 and 5 on both ends if DB25 to DB25.
If you want a for sure solution, order one of our cables and if it does not work, I will take it back and pay for the return shipping. Over 60% of our technical calls are because of crappy cables and wrong pinouts.