How to test Bit-Banged communication's assembly routines

For one MCU I have written some assembly routines performing RX and TX of a proprietary protocol (UART-based) in a bit-bang fashion. How can I test them?

TX might be tested by sending data, and at the same time, with the help of a logic analyzer, checking that all the sampled timings are correct (manually or with some scripts).

RX on the other hand is more difficult. On one hand I can check if I'm receiving what someone else is sending, but on the other hand how do I know that the RX sampling is happening correctly (timing-wise)?
For example, my RX routine may return the correct data by sampling at the edge of the "bit window" instead of the middle.
I thought about toggling a "debug pin" to indicate when the sampling is actually happening, but this introduces delays in the sampling procedure, hence I wouldn't be testing my original routine.