The simplest and most useful fitness test is a time trial. Pick a local hill and time yourself up it. Record your time -- and your average heart rate if you're using a heart monitor. Repeat as often as you want. That's what I do.

I maintain a chart of climbing rate (ft/hr) divided by heart rate (beats/min). The values move around quite a bit, but I can get a sense of the trend.

I recently got the flu, and my fitness was way down when I got back on the bike. I rode every day this week, charting my progress on a short climb. The daily gains were substantial, and now I'm back to my pre-illness levels. See the chart below (the horizontal line is the average for the year). Since the chart tells me I've recovered my fitness, I can now start doing more intense training.