I use a board located at the point where I plan to attach the hinges to the lid, and a a bathroom scale underneath to measure the force.
The photo above was taken when the lid was framed but before I added the plywood skin. The scale shows 42 pounds, and I estimated another 25 to account for the 1/4" plywood that would go on later. So two struts of about 35 pounds force each would the bare minimum required to support this cover (this is a lightweight cover design). It would be good to have struts of at least 10 pounds or more higher than the minimum required for practical use.
Plywood on, struts installed, but before the lid was painted:
I used the same technique to measure the force necessary on my military trailer. Many people put their struts on the latch side of the lid, I put mine on the hinge side so they don't obstruct access to the inside as much.