A cabinet leak needs a dry starting point and one controlled observation at a time.
The short answer
Remove belongings without disturbing pipes. Note wet valves, supply hoses, faucet tails, basin connections, trap joints, disposal, and cabinet seams. If water is active, stop fixture use and operate only a known safe shutoff. Avoid testing near wet electrical parts.
What to notice
Supply-side signs
A valve or hose can leak while the faucet is off, only while it runs, or when a connection moves. Record the timing and whether closing the fixture valve changes it. Do not force a valve that will not turn normally. Corrosion, staining, or mineral deposits show an area to inspect, not necessarily the current source.
Drain-side signs
Fill only enough to reproduce a safe observation if the cabinet is dry and there is no hazard. Watch the basket, disposal, trap, and wall connection as water drains. Stop at the first visible leak. Do not loosen a trap containing cleaner or unknown wastewater, and disclose any drain product to the provider.
Cabinet and wall clues
A swollen base, damp rear panel, or stain from above may show that water has traveled or the leak is older than first noticed. Ask who assesses and dries those materials after plumbing is controlled. A dry surface after wiping does not prove the cavity or flooring beneath is dry.
One useful question
Can a paper towel locate the source?
It can help reveal new moisture on an accessible dry surface, but it does not make electrical areas or unstable connections safe. Use it as an observation aid, not proof of the underlying failure. Stop before moving or disassembling anything.