Four Things Leak Into the Same Cabinet

Dry everything, then wait

Dry the cabinet floor completely and lay dry paper towel under each candidate separately: under the two supply connections, under the shutoff valves, under the tailpiece and trap, and under the faucet mounting.

Then use the sink normally for an hour and look at which towel is wet. That is the whole method and it beats guessing.

Wet only when water runs down the drain

Then it is the waste side: the tailpiece, the trap, a slip joint or the connection to the branch. Drain leaks are not under pressure, so they appear only when water is passing, and they can be surprisingly slow to show.

A basket strainer failing at the sink bowl behaves the same way and is often mistaken for the trap.

Wet all the time, running or not

Then it is the supply side, which is under pressure whether or not anybody is using the tap: a supply connection, a flexible line, or a shutoff valve weeping at its packing.

This is the one to treat as urgent, because a pressurized fitting that has begun to weep can let go properly, and unlike a drain leak it does not stop when you stop using the sink.

Wet only when the faucet runs, from above

Then look up. Water arriving from the mounting holes is the spout seal running across the deck and down through the fixture, and the cabinet is just where it collects.