You will notice a drop but the discharge curve of a lipo is so steep that even if you drive the car back to the as soon as you notice a drop you can ruin the battery.

What I suggest is that you time your runs and read the voltage after. Keep track and every day run 1-2 minutes longer until you read 6.6v (don't try to use 6v as a guide, that is the ABSOLUTE minimum the cells should be).