Consider using a scope to ensure analogue signal integrity. i.e. voltages, timings, transition time/monotonicity/overshoot, noise. Once you have determined all those are within limits, you will be sure the analogue waveform will be correctly interpreted by the receiver as a digital signal.
At that point flip out of the analogue domain (i.e. a scope) and into the digital domain, i.e. a logic analyser for bits/words or protocol analyser for messages.
Very capable LAs and PAs are very cheap, and remarkably effective. Examples: the many 8-bit 24MS/s "LAs", or a BusPirate 5. The latter will allow you to output UART/SPI/etc messages as well.