How do we know the receiver delay in an OFDM transceiver system?
In basically all simulations I've done, you just send known (but, depending on what you want to simulate, seemingly random) data. Then you correlate the received bits with the transmitted ones, and look for a peak.

That's your end-to-end bit delay, which you don't really care about, but it's how you know which bit you compare to which.

If you data has no significant autocorrelation, and assuming your transceiver basically works with a BER much smaller than 0.5, this is pretty reliable.