If you are curious about your current fitness level, there is no better way to evaluate your performance than accumulating data using a Power meter. A Power meter will give you definitive answers as to your actual effort measured in watts and the related response that the body has towards that effort by means of your heart rate. In addition to those two variables (watts & heart rate) you can also get valuable information as to what type of riding style delivers the most Power by taking note of the cadences used for different levels of effort.
The true beauty of evaluating your performance based on Power numbers is that it removes any environmental influences (temperature, wind, terrain) from your effort and gives you a clearer picture of how well you actually performed. Below I will give you an example of the two different testing approaches, one using the typical cycling computer and the other using a Power meter.
Let's say you periodically tested your conditioning levels by going out to a familar 3 mile stretch of road and perform a personal time trial. You then evaluated your performance by recording your average mph, riding time, average heart rate and then compared it to a earlier effort on the same 3 mile course. When you compared the two samples you noticed that the most recent 3 mile time trial was 20 seconds slower and an average heart rate 3 beats higher than the previous effort from a month ago. You might conclude from this comparison that you didn't have as strong of a ride and maybe you should intensify your training to gain back the fitness that you believe to have lost.
Now lets look at the scenario above except that you have a Power meter on your bike which gives you average watts produced and wind speed in addition to the variables that the typical cycling computer gives you. You again rode the same 3 mile course with a 20 second slower result as in the above example, but when evaluating the data you notice that you produced an average of 250 watts compared to just 240 watts from the previous month. The reason for the slower time in this most recent time trial effort was due to a little stronger headwind that was present on that particular day, but by having the Power data available, you are able to conclude that you actually rode stronger, and everything is right on schedule with your training.
You can see from these two examples that you can walk away from a fitness evaluation with two totally different perceptions. In one, you are discouraged about your effort and in the process create self-doubt about your abilities and your training program. In another, just by gathering more information, you are pumped about your effort and walk away with a load of confidence in your abilities and your training program.
To fine tune the testing process even more, I recommend creating your very own testing laboratory by utilizing an indoor setting where you can eliminate all the environmental variables from your comparison of similar efforts.