"I find it much easier to test it on a long, steady climb, 20 minutes or more. If you have such a thing near you, have a go. The fastest pace that gets you to the top without blowing up is at least close."
That's exactly the way to do it. I had LT test done once when Cadence Cycling offered a deal on it at the LBS. They got 158 and the way it was done (power increments a little coarse) probably underestimated the LTHR a little bit. Prior efforts on long (30 minutes or more) climbs were suggesting 160-162. If you find a long enough climb, 20 minutes or more, it is almost sure to get your LTHR to high accuracy.