If you are trying to find out where it shoots, it's hard to beat 13 yards or so, off a rest, several shots. Free-hand is a waste of time and shells.
If you are trying to find out _how_ it shoots, you are pretty safe just assuming it shoots OK and leaving it at that. Really finding out the "quality" of its patterns is more work than it's worth and, generally, you can't find out anyway.
treefrog, here is what I would do with any new gun, especially one that used interchangeable choke tubes.
Set up on a bench and a rest in front of a grease plate or target placed between 13 and 20 yards in front of you. I actually do this at 20, but that's for convenience. 16 is fine. I find 13 a little close.
Pick your tightest choke tube and put it in one barrel. Sight that shotgun like a rifle and fire at the target enough times that you can say with precision where that choke prints. Now do the same with all your other choke tubes. Weed out the ones that do not strike the same POI. When I did this with my 682 Gold E combo, only two of the six tubes supplied shot to the same POI.
Now take one of the "good" tubes and put it in the other barrel. Fire at the target and determine whether the two barrels are properly converged. If yes, you are good to go. If no, you'll have to decide what to do about it.
Next, set the gun to shoot a little high, say about 2" high at 20 yards, or the equivalent at shorter ranges, and have your wife shoot it. It won't take long to figure out if the POI needs adjusting.
Once you know your chokes/barrels shoot straight you can decide whether you want to pattern.
Don't listen to Neil. Test the quality of the pattern by shooting at least 10 patterns for several different loads. Then spend a couple of hours counting holes and apply some basic statistical analysis to your data. After you have done that, then listen to Neil.
Some tips- Shoot several patterns at 13 yards to determine the point of impact (not pattern quality). This is important. Then after your wife gets to the 25 yard line, do some pattern quality testing using Green Dot powder and explain to me why you will get 4-7 more holes in the pattern core with this powder.
bluedsteel, if you do not first do what I described above, you will not know what you are working with. Once you know your gun shoots straight and so do all the chokes, then you can go out and find out where you shoot it. Some time spent at 15 yards in front of a pattern sheet will save you a boat load of time on the line.
Once you know your gun shoots straight, and you shoot it straight (especially left-right), you can work out your effective POI on the line shooting targets. If you haven't done the two preliminary steps first, that can be a very, very frustrating process.
I will never forget a shooter who couldn't understand why she kept missing left angle targets with her new Browning. (20 yrs ago) Another shooter took her gun and patterned it and found it shooting about 12" to the side.
Shoot a few papers from a bench rest at 20 yds. for impact point, then forget about it. You can get into the pattern details, but you'll know best by how you break the targets. Choke tubes do matter, Wright's tubes have excellent annular (outer) rings and are very even. Ljutics, for example, have tight cores. Remember the shot string importance vs. paper pattern. Years ago I patterned and chronographed about 125 rounds. Powder, speed, wads, hulls, primers and choke tube mfgs. all make a difference. Find a comfortable load, shoot it, and tighten to a smaller bore if you think you have holes.