For checking it, if you disconnect the pump end of the pipe and pour water into the pipe, it should stay full once you fill it up and get any bubbles out. If it drains down, something is leaking, with the pitless O-ring and the foot valve as prime suspects, but some other hole in the pipe for some other reason is not impossible, just less likely, by far.

<s>It might be a bad/leaky foot valve. At all of 25 feet, screw a pipe handle into the pitless adapter and yank the pipe, assuming your pitless has the typical pipe thread connection to put a handle on to yank it, as mine does. You can inspect the pipe to see where the water level is by where it's wet when you haul it up. You can inspect the foot valve for any issues and replace if needed. You can also inspect and/or replace the o-ring in the pitless adapter, as it's normally in the part you pull out, for hopefully obvious reasons.</s> That seems less likely with the problem description in the comment above.

At 25 feet, you should also be able to shine a flashlight down the well and *see* the water level, and/or lower a weight on a rope to measure it. Do be careful about **not** dropping things into the well, and don't leave it open and unattended.

It's certainly possible that your static water level has dropped, or your recharge rate (how fast water comes in when you pump water out) has. One way to check on that would be to put a vacuum gauge in right before the pump, and see if the reading indicates more and more vacuum as the pump runs, which would be associated with lower water level as you pump the water out. Some increase is normal, but generally it will stabilize if the recharge rate is adequate. If it simply indicates more and more vacuum until the pump craps out you might be overdrawing the well.

An alternate possibility for some shallow wells is that the well has been infiltrated by sand, and the screen on the foot valve is loading up - but I'd expect you would have some evidence of sand making it into the pipe as well, if that's the case. You can make use of a weight on a rope to check both the water level and the present well depth (lower to where you see/hear it splash, and then lower until you feel the weight hit bottom.)

There are a few limited issues with the piping you might be able to solve yourself, but in most cases if it's a "the well itself" problem you'll need a well driller.

After opening the well, remember to sanitize it when done.