How to Determine a PCB's Power Requirements?
You can measure the amount of current the board draws using the current measurement capability of a multimeter. If your multimeter does not have a fuse on the current input you should put one in series. You place the multimeter in series with either the positive or negative line of your power supply. I would use the negative line.

When first powering a device I would use a supply with a current limit. This limits
the current if there is a fault or if the vendor really meant 5V and not 12V, etc.

Did the vendor mention if the device has an internal current limit?

It is not clear what the "high voltage" protection is. It could be a transient protection
device that will not protect the device if you apply a continuous voltage that is
beyond the input voltage range.

It is not really practical to determine the input voltage specification unless you
have a reasonable idea what it is before you start. You will not be able to determine
the maximum input voltage reliably.