I haven''t found a very reliable way to quickly test pointing ability on a boat that I have not sailed enough to know how to dial in (by which I mean optimize sail trim and steering for the conditions).If the boat is instrumented with a compass, masthead fly and knotmeter, it is a little easier. The difficulty in getting to an accrate assessment has never stopped me from trying.
When I want to evaluate pointing ability on a strange boat, I typically spend time getting to know the boat and figuring out how to get the boat dialed in on one tack by experimenting with sheet leads and tension, watching the knotmeter and masthead fly, looking at the position of the jib relative to the lower shrouds and at spreaders, and tracking the general relative wind direction on the compass. Over time I begin to get a general sense of the proper set up to optimize pointing ability on that tack. I do this again on the other tacks because many boats are assymetrical in performance or instrment readings, and because sailing conditions such as current may effect the settings on one tack versus the other. Once I feel like I have an understanding of the settings that optimize the pointing for the given conditions I sail on a tack until dialed in to maximize VMG, then tack and sail on that tack until dialed in, and then go back again, over and over again. With each tack I record the sailing heading from the compass until I have a sense of the relative compass courses between tacks.
Of course that is only a part of the story. Being able tack through some given compass angle is a big piece of the puzzle but so is leeway. many boats can be pinched and achieve reasonable speed and narrow tacking angles but are making so much leeway that the VMG achieved is slower than pointing a few degrees lower would achieve. I check leeway in a number of ways. The quickest is to simply back sight your own wake. If you are steering a straight course, you can usually see the angle of the wake relative boat. This is not a particularly accurate indicator since wind driven surface current can come into play here, but if you get into the habit of back sighting your wake on a regular basis, you develop a sense of what larger and smaller amounts of leeway look like.
Another way to evaluate leeway to sail up to another boat from astern (Far enough back to avoid their dirty air). By sighting over that boat to an object on the horizon you can get a relative sense of that boat''s leeway relative to your leeway. Again, the problem is calibrating what you see, and again, if you get in the habit of doing this on a regular basis with a boat that you are familiar with, you begin to end up with a kind of mental database of how different boats behave in terms of tacking angles and leeway.
Of course the best way to see how well a boat points is on the race course. There you typically have crews and skippers who have a reasonable understanding how to dial in their boats and they are sailing in a pretty confined area so that you can watch trends over the course of the windward leg.