A 1961 model year should have come with the B16 and 4.56 rear end. But the cars are getting a little old now (ahem) and there has been ample time over the last 5 decades for people to swap parts around. And the supply of B18 and B20 motors is vast compared to the supply of B16 motors. And the parts supply situation is similarly more difficult for a B16.

Already mentioned, but with some visual aids here. The B4/14/16 valve cover, easily seen in even a tiny blurry picture in an ad (if they have a pic of the motor) is rectangular, square-ish curved edges, with some creases in it. It's held on by two nuts through the top side of the cover.



The B18/20 valve cover is trapezoidial, has rounded edges, and no creases or grooves in it. It's held on by 4 screws on its lower edge:




Additionally, a '61 PV would also have had a 6 volt electrical system. But it's not that hard to convert a 6V PV to 12V, mostly because the electrical system is so simple.
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'63 PV544 rat rod, '93 Classic #1141 245 (now w/16V turbo)

I think even easier to ID the engine...
B16 has the starter on the right side under the carbs.
B18 is on the left side under the distributor.

Pretty good chance that the B16 has a 4.56 rear and B18 a 4.10.
Easiest to look at the tag as suggested, but you could always raise the rear end, block one wheel and count how many turns of the companion flange for one wheel rotation.

Two easy and obvious ways to ID the engine. First, under the #4 spark plug there is a casting ID "B18". Second the B18 has a Thermostat housing at the top front of the head that points straight up. The B16 points forward.

With "PV's" of this vintage it is hard to guarantee that the original factory installed engine is still in use thus you will need to check. For further identification there are "stamped" numbers on the left side (Distributor) of the block that further ID the B18 by various model configurations.

To check the Rear Axle, you will need to look for a small metal tag held on by one of the bolts on the "pumpkin". It will have the gear ratio stamped on it.