This is not rocket science. It is easy to do with another hand bearing compass.
1. Look around your compass and check to see if there are any significant steel or iron items nearby (consider pots and pans under the bulkhead where the compass is mounted.) If so, move them and note whether or not the compass needle moves at all and how much.
2. Take a bearing on a distant object with the boats compass (head the boat directly toward the object and check bearing).
3. Get as far as possible from your iron keel by standing on your boom and take the same reading with your hand compass. Try leaning way back off your stern rails and do it. Any Diff tween that bearing taken in step 2 and this one? If you cannot see any difference, then your deviation is too small to worry about.
4. IF you see a diff (say > 5 degrees) then you need to take addtl steps. Note on your chart the actual bearing from one known nav aid buoy to another, compare to to what your boats compass says. Subtract (or add) known variation printed on your chart and compare again, the diff is your deviation (for that nearing). In general, at low latitudes, the diff in deviation between any bearings will be small.
In almost all cases for a fiberglass sailboat with no ferromagnetic items near the compass, the deviation will be small, even with an iron keel. The VARIATION can be significant. In the Abacos, it is about -10 degrees meaning you have to add 10to what your compass says.
Thanks. Lots of good replies here. Will construct a deviation chart over winter for use during spring and summer voyaging.