Tach indication at known speeds should be a good option other then the tachs in these cars often have errors. Typically the tach error will be to read low so a DX transmission could give tach indications that look like an HF. An HF transmission should give you about 2100 to 2200 (depending on tire size) at 70 MPH in 5th gear but like I said there are likely tach errors.
For positive identification you need to determine the gear ratio by comparing turns of the input verses output shafts. See this thread for gear ratio information. viewtopic.php?t=246
With the transmission in the car the best way to do this will be with one front wheel raised off the ground and the other wheel blocked. When doing this the differential will double the turns of the raised wheel so be sure to take this into account when comparing turns.