Looking at the PPM pulse will not tell you signal strength. The encoded signal is substantially conditioned at that point and you will only be able to determine when the signal is corrupt or missing. This is the equiv of detecting that you are low on fuel after you run out of gas.
If you want to know RF signal strength then that is another matter. Some FM Demod IC's have an RSSI output for this purpose. You will need to look in the Rx and post the part number to the FM Demod IC. If you do this then either I or someone else will be able to offer some help.