How to measure recovery?

[Lewis Moon]

So...how do you measure recovery?

I ask this because, at 56, I'm kinda struggling with being able to (semi) accurately judge my recovery. As a kid I'd run two workouts a day during X-Country season, but I know I need a lot more recovery time now.
In the past, I've had more than a bit of a problem with spontaneous combustion due to over training. I'd like to avoid that, but I get really ancy when I start to feel good. This happened earlier this week. On Tuesday I was scheduled for FTP+ so I really lit the torch and took 1.5 minutes off my PR on a local 5.5 mile hill climb. I couldn't see my Garmin so I wen't out a bit fast and ended up posting my best Power/HR numbers ever, by a long shot. For the rest of the day (at work) I really felt fried. Duh. That wasn't really the problem...what came after was.
I have a 3 day rest scheduled over the long weekend so I went ahead and did a tempo trainer work out on Wednesday afternoon and then, with rain threatening to wash out Thursday and friday, I backed that up with a tempo ride on Thursday morning. I really felt like I had nothing in the tank. No snap, no sizzle. However, I didn't feel too bad before the ride...at least not bad enough to not take the opportunity to train.
The weekend is (almost) here so I'm knocking off for a couple of days...but back to the original question: How could I have measured my recovery and avoided the crappy and (probably) counter productive workout?