Depends on what kind of washer is involved -- topload, frontload, brand/model -- which isn't specified in your question.

fl bosch

I'd let the wash cycle begin at the set temp. Then I'd do a cancel and drain and use a meat thermometer at the discharge. If your discharge is sealed I'd use an infrared thermometer gun on the output hose.

output meas drain hose?

will it not get colder by the time it reaches the drain hose?

will an infrared thermometer against the glass door work?

nope cannot be stopped when you pause/stop it goes into cooling mode before you can open it

I have a Duet. This is what I did. On the Allergen cycle which has a suds cool down before draining I cancelled the load just before the water started going in for suds cool down..it instantly went into drain which took seconds than the door unlocked. I opened it and shoved a therm into the hot, wet, steamy clothing. I've had 158F and 159F so I'm guessing maybe a degree more. For my other cycles I watched the clock and when I knew the wash cycle was about done I did the same as above. Its not perfect but gives me at least a close ball park figure. Whirlpool only releases temps they want us to believe which in my case was wrong.

Check your owners manual. Back when I had my Bosch , hot was 120, warm,86, jeans85, kids care 158,sani 170 and cold was 60 something. Those temps. are all I miss,well that and the water usage, about that machine.

ETA: I was always able to pause mine, except on sanitize. And I almost always added more water, 3 or 4 gallons to the wash cycles, but it still used way less water than the SQ.

This little gadget:

will give you the temperature of any surface in a fraction of a second.

It is frequently a "loss leader" at Harbor Freight and Amazon. Today was $17 at Amazon a few minutes ago.

However in the case of laundry you have to get the door open. It won't work through glass.

cannot open the door in time that is the isue