How do you know IF you should train???

I did get the third day of the third week of the Neupert program in on Saturday but something has popped up since then that has put a little halt to my training and brought the question above to the front of my mind...

Sunday night my Right big toe was a little sore but no big deal but by mid day on Monday it was a big deal - very painful and swollen and warm to the touch and a very altered gait.  

Even worse this morning so I am off to the Dr. tomorrow to get checked - will report the findings.

So now to the question - 

How do you know IF you should train?

I could go ahead and swing and "workout" because I can stand in one place - HOWEVER - I tested my Straight leg raise and my Rolling Patterns and they are altered from what is normal for me.

Meaning it is a VERY poor idea for me to train today!

This is just one of the benefits of the FMS and having a consistent repeatable assessment.  I could have convinced myself that it was ok for me to train but the screens don't lie!

So how do you know if you should train on a given day?

Also- the next question that should pop into your head if you have an ache or pain is - how is it affecting you?  How would you know?  How would you know if it is resolved and that it is ok to go back to training?