This can get difficult.
1) The boat has to be level. Get a spirit level. Pick a dead still day. May not be possible for your locale.
2) When level, put two pieces of heavy tape across the partners. Make sure they intersect dead center in the partners' hole. Use a ruler.
3) Drop a heavy thread with a lead sinker on it to just short of the mast step, and I mean 1/4"/0.5 cm.
4) Get someone light, like a little girl with her wits about her, to go below and put a dot with a felt pen directly under the lead sinker...BUT make sure the boat is still perfectly level, which may involve two spirit levels (fore and aft and port and starboard) at right angle (use the partners, plus a pair of opera glasses to READ the levels as you move around the boat to achieve levelness.
Then you've got to wait for the pendulum you've made to stop swinging and start just hanging.
However, if you do these things, you will have the means to determine easily how aligned your partners is to the step below.