I just wrote a long, detailed response; hit “submit” and lost it.
Using the HR drift article on this site as well as Scott Johnston’s post here, you can very easily do a HR Drift test without TP.
1. Do it only when healthy and well-rested, preferably after a rest day.
2. Only do it with a chest strap HR monitor. You will be wasting your time with an optical sensor.
3. Take the average HR of the second 30 minutes and divide it by the average HR of the first 30 minutes. If the result of that division is 1.05, then your average HR for the first 30 minutes was your AeT HR.
4. Results will vary day-to-day.
I have done this without TP a number of times on a treadmill.