Original source: http://lists.topica.com/lists/wattage/read/message.html?mid=910290920
Andy Coggan wrote:

..er, ways of determining your functional threshold power (roughly in order of increasing certainty):

1) from inspection of a ride file.
2) from power distribution profile from multiple rides.
3) from blood lactate measurements (better or worse, depending on how it is done).
4) based on normalized power from a hard ~1 h race.
5) using critical power testing and analysis.
6) from the power that you can routinely generate during long intervals done in training.
7) from the average power during a ~1 h TT (the best predictor of performance is performance itself).

BTW, another method that could be added to this list would be to do an incremental exercise test to determine 'MAP', then estimate functional threshold power as being ~75% (range 72-77%, using Ric Stern's guidelines) of this value. You could then use this estimate as is, or if necessary/desired, further refine it using one of the methods described above (e.g., by doing a TT).

BTW, the reason this approach works is because in trained cyclists, LT falls within a fairly narrow range as a percentage of VO2max, and there is tendency for those with the highest LTs to have the lowest anaerobic capacities (and therefore a slightly lower MAP relative to power at VO2max), and vice-versa. In any case, at the very least knowing your MAP will help 'bracket' what could be considered a reasonable range into which you expect your threshold power to fall.