Before extending the trailer jack to raise the coupler off a tow vehicle's pintle hook, what must the driver check?

AAMVA CDL Manual, Section 6.4.4 "Uncoupling a Pintle Hook," Step 4, p. 6-11

If the ground under the jack pad is soft, uneven, or otherwise unable to bear the tongue weight, the jack can sink or tip once it takes the trailer's weight, dropping the trailer unexpectedly. Checking ground support first prevents an unstable, potentially dangerous uncoupling.