You really can't measure either J or I. I guess it is technically possible, but it would take a precise tape, a good understanding of trig, and plumb bobs. Plus there is very little reason too. Unless the boat is a home design, the specs should be available from the designer
, and there are a number of places online that have databases with this information.
That being said, to measure the J the easiest way is to first remove any pre-bend in the mast and ensure it is perfectly verticle. Then ensure the boat is floating directly on its lines, and is balanced for and aft. Then draw a line vertically from the bow chainplate, parallel to the mast. Finally measure the distance between the two parallel lines.
Of course you have to ensure that the mast is in the reference position both on the mast step and at the partners, which requires a call to the designer
to figure out exacally where they are. And while you have him on the phone
, you might as well ask about the designed J leingth too.
To get the I, once you have the J, just draw a triangle. Then measure from the bow chainplate to the genoa halyard
. This is the hypotenuse of the triangle. The J is one leg, the right angle is the one between the I and the J, then a little trig will work it out.
Or go to sailboatdata.com and look it up. It's a lot faster.