Take a look

I say just in time. When in doubt clean it out.

If you see this mess I don’t think you have to explain to the contractor

For those times, story of our lives, we're waiting on mud i make it a habit to know....

1. 1. The Mix design
   

2. Batch time of Mud in pump

3. Temperature of Mud in pump (anybody whose pumped a decent amount of concrete can tell the temp. by feel)

It is inevitable, my time will come, but in my 2.5 years pumping  I've been fortunate enough to avoid that 

This can easily happen if the concrete flashes, hate having to wash out mid job but its better than this heh. Like you mentioned the concrete temperature is a good signal of what's going on.