Have not ran the washer for several days now. Concerned it could have frozen as it’s on exterior wall. What should I do before starting a load to check?
Are you talking about the drain? If not, then you are already past anything you can do to prevent a problem. It is under pressure and just waiting on a thaw to do whatever it is going to do. If you are worried about a split pipe then you probably need to turn off the whole house and wait until you are confident that it is thawed.
To know ASAP if there is a leak you could watch the meter. Assuming all the rest of the lines in the house are full, as in use now, then when you turn the water back on at the meter just watch it. They are highly sensitive. It will move a little just to pressure up, but should stop. If it keeps moving shut if off. Houston you have a problem.
If you are concerned about a P trap in the washer drain then just pull the washer out and pour some water in the drain from something like a 2-litre bottle and listen for it to fill up.