Deck screws undetectable and boards are rotting
The deck screws are most assuredly made of steel which has iron in it. It should be very easy to find the screw locations using a magnet. 

I have used these inexpensive type of magnetic nail/screw locators on walls and ceilings. I suspect they would work well on your deck as well. 



(Picture Source: https://www.amazon.com/Stanley-47-400-Magnetic-Stud-Finder/dp/B0000BYD3K)