Well you know that the head is a PR-3, so its more than likley an ITR.
You know that the block stamp is a B18C, JDM engine.

You could check the ECU and verify that its from an ITR. Then run a compression check like said above. Other than that, the only real way to tell is drop the oil pan and have a look at the crank.

The only possibility that I see here is a "poor mans ITR", which is a GSR block and a B16 head. Is the seller claiming a full swap, including LSD tranny? Are there any mods to the car?