Is your training working? The only way to find out is to test and retest yourself to measure your progression.
You need to know what you are testing and why for the data to be useful! A six second sprint test might not be as important as an ftp test for a time triallist, and vice versa for a track sprinter.
Testing with a power meter is definitely the way forward as it gives the most accurate and informative results. Remember to recalibrate your power meter though, before every test (I usually recalibrate before every ride).
Indoor tests on ‘Smart trainers’ are usually best as you can ensue all other variables are the same for more accurate results. For instance the wind and weather conditions don’t come into play, so can’t skew the results.
Then again a good field test can give you a real insight into how you perform in real life, which can be very different from our performances in the sterile environment of an indoor turbo set up or lab. Even without power meter you can get some really useful information about how you are improving as long as you think hard about reducing any random variables such as wind strength and direction. Testing up a sheltered hill, for instance, or on a circular loop, can keep these outside variables to a minimum and so provide you with a good indication of how you are improving out on the road.
Interpreting the test is not just a case of reading the data. A good coach will be able to tell you so much from the test than just the raw numbers. They will be able to give more of an in depth analysis and importantly tell you what you need to work on, how to improve it and what this means in terms of real world performance.
Here is a great video that explains some of the basics of fitness testing.