How do I know if I'm overtraining?

I just want to get some advice. I've seen some training plans for bike racing out there, and considering that I'm no professional, I've had to modify it for my lifestyle. Basically, I work out 6 days a week. 2-3 days I do interval training/weight lifting. 1-2 days I do yoga and spinning. I do 1 long bike ride or race 1 day, and I bike commute 4 days 10 miles at a good pace. Somewhere in there I try to do a day of nothing, or just the commute.

Do other people know if it's better to plan more rest into training? Also, with training that is heavy on certain muscles (like quadriceps) any way to balance that?

Please feel free to share your personal experiences with your own work out regimens and/or training schedules.

Thanks!