It is difficult to nail it down, unless it is an obvious rust problem. I would recommedn that if you do not wee any obvious rust under the cowl that you simply replace the windshield seal and see if the problem goes away. Believe me, you will not be able to directly find and fix the leak. Water can only go straight down. It canot go right, left, or up. Therefore, it is impossible for water to come in through the corner of the windshield (usually). In fact, if you replace the windshield seal take a look at the ledge that the windshield sits in after you have it out. Yo will notice thgat there are two drain holes, one on either side, to allow water to drain from ther bottom if this ledge. Water that leaks in around the bottom corners of tghe seal will simply drain through this hole and harmlessly drain into the cowl. From thee, it should go into the drain hoses and down to the fenders, rusting them out over 10 years. If you look at it and remember that water can only go down, you will see that it is impossible for water to come in through the borrom corners, due to this thing called gravity.
What is likely happening is that water is coming straight down from the top of the windshield cowl and coming in from the sides of the windshield around the windshield pillars. When yopu have the windshield out you will see that that is possible, gravity-wise. You will see a trickle of water when you put a hose to it, but it will be hard to see where it is coming from.
This was my experience on my 78 280Z. Problem was solved. Good luck.