Do you ever wake up, or finish work and think "oh god, there is no way I have any energy to go to the gym?".
Sometimes you don't. Sometimes it is a better idea to go home and rest or meditate, rather than push through. Sometimes you haven't slept enough, eaten enough, or relaxed enough and your body is DONE!!! On these days it is important to honour this, and go home and rest!! If you continue to train when you are exhausted and stressed you are only stressing yourself out further, and potentially damaging your body.
Other times, you have mentally talked yourself into being tired, and this is a different story!
So, here is how I figure out if I am tired and it is better to rest, or if I have some hidden energy.
First: Do a short warm up- 10 minutes, go for a jog, and do a good solid dynamic warm up. https://www.youtube.com/watch?v=hGs7aexZQ-I
Then, hop on the treadmill (or machine of your choice) and do 10 minutes of some kind of intense cardio. For example, I do 1 minute of sprints (level 8-10) followed by 30 seconds of walking, for 10 minutes.
If you are SUFFERING through this cardio, your legs won't work, you feel like you want to die, your body feels tight and awful, it probably would be more beneficial to cut it there and head home.
More often that not I turn my tired crappy energy into a FANTASTIC workout!!!