How to measure the temperature of a pan?
You can get a remote-sensing thermometer (they have a laser that shines on the object to show you where you're pointing it). They read reasonably accurately without touching the heated object, and come in various sensitivities. Mine reads as high as 700 degrees F, but there are cheaper ones that don't go as high.

Apart from that, I would think you'll always be just estimating and not very accurately at that.

Don't forget also that the temperature of your cooking surface must be higher than strictly required for the Maillard reaction. If the meat needs to get to 155C for it to occur, you will probably need to heat your pan higher to account for heat lost to the meat on contact.

And for killing bacteria, you may also need to get the entire piece of food to the required temp, so a regular food thermometer is a better bet than trying to measure the heat of the pan.