How to Identify Mental Fatigue vs Physical Exhaustion

I woke up today and I just felt heavy. Not like I am sick, but like my brain is tired of the grind. I am trying to figure out if I am actually physically exhausted and need a rest day, or if I am just being mentally weak and need to push through.

It is a fine line to walk here in Edmonton where the weather is often gloomy. I do not want to skip a session if it is just a lack of discipline, but I also do not want to cause an injury because I am not focused. How do you guys tell the difference between the two?

Do you have a specific test you use? I have heard that if your grip strength is significantly lower than usual, it is a sign of physical fatigue. I would love to hear your methods for evaluating your readiness to train when you are feeling low on energy.