Professional word meaning to take a reading from something

I think the idiomatic way to say it, in this context, is gauge.

1. estimate or determine the amount, level, or volume of.

Determine

to ascertain or conclude, esp after observation or consideration

or

Estimate

1. To calculate approximately (the amount, extent, magnitude, position, or value of something).
2. To form an opinion about; evaluate: "While an author is yet living we estimate his powers by his worst performance"

(both definitions from American Heritage via TheFreeDictionary)

While the electricity guy can accurately take a reading (and he gets a specific number from it), "demand" is not so specific. The electricity guy can "estimate" consumption for the month ahead based on what he has "determined" from past observations.

For what it's worth, I like gauge as in other answers but I naturally avoid it because the spelling of it is unintuitive for me. I always overthink my pronunciation of it.

Measure

What you are doing is measuring the value. Therefore, measure is the correct word.

While I like gauge, I'm going to suggest another possibility anyway.

(I only just realized that it's referenced in another answer—which mine references in return. So, take your pick if you like either one. . .)

By carrying out small-venue product launches one can monitor demand.

verb

: to watch, keep track of, or check usually for a special purpose · Nurses monitored the patient's heart rate.

noun

1 c : one that monitors or is used in monitoring: such as (1) : an electronic device with a screen used for display (as of television pictures or computer information) (2) : a device for observing a biological condition or function · a heart monitor