If you have a suitable turbo trainer, you can gauge improvements over various durations by simply doing a maximal effort over said duration and noting the average speed. For this you do not need anything other than a (suitable) turbo trainer and the same set up procedure each time. Provided the turbo is as I said, suitable i.e. it has a repeatable resistance curve (power vs speed) then an increase in average speed will indicate an increase in power. You won't know how many watts you have improved by, but you will know you have improved and be detecting fairly small, realistic gains.
If you don't have a suitable turbo trainer, you can't really do this, except to detect gross, unrealistic or long term (large) changes since any indication of improved fitness over any sort of short term (small changes in fitness) may well be hidden within the noise created by the unrepeatable resistance curve.