1) The gold standard is the tried and true hour of power, the 40k time trial. Going as hard as you can for an hour. Paced properly, this will most directly reflect your FTP.
2) Use a laboratory quality gas analyzer with blood lactate testing. In our facility, we use the CardioCoach gas analysis system and take serial blood lactate measures. Accurate, but having a skilled coach locally to administer the test and interpret the data in most cases is not available.
3) Taking your normalized power (NP) from a race or hard group ride over a period of 60 minutes. Normalized power is a calculation that factors in the power you could have maintained if you were riding at a steady effort. If this number is bigger than the one you are using to train, your FTP just went up!
4) Computer modeled FTP, using tools like Training Peaks’ WKO5 software, can compile your data and give you a reasonable estimate of your FTP. If you are skilled with data analysis or have a skilled coach to interpret your data, this can be an effective way. I use this with my athletes and cross check with field tests.
5) The method that is most often used is a 20-minute test then multiplied by 95%. This often over-represents your FTP. A more accurate representation of this test is to perform a 5 minute ALL OUT effort with 5 minutes rest BEFORE your 20-minute test which will better represent, but still, is not as accurate as the hour of power.
6) The critical power method. Taking maximal tests over the course of a week from efforts of 3 seconds to 30 minutes in duration. Graphing the data and then adding best fit lines to the graph. Time consuming and it takes a substantial amount of time and maximal effort testing.
7) A histogram, or basically a graph of all the power numbers that you ride in regularly. The highest level before you take a sudden drop in power ~ 5-10% would be roughly your FTP.
The important point here is that the test is hard and if you select a shorter duration it is difficult to get an accurate measure of your FTP, your efforts should be routinely re-tested every 6-8 weeks for improvement and to make sure you are training off of the right number. In my opinion methods 1-5 (with the exception of #2) would be your best options to get a good field test.