How to confirm screw is attached to stud through wood plank and dry wall?
I suggest you keep marking the centers like you've done and as each plank goes up, copy that mark to the face of the plank.

You can use painter's tape if you don't want to make marks on the planks. At minimum mark the entry and exit points of the projection of the joist across the face of the plank and make a line between them (a first order approximation). You'll hit a joist most of the time. If you want to be able to better detect a joist hit, then pre-drill holes in the planks or use partially threaded wood screws with a shank longer than your plank depth.

You may not hit a joist every time, but I really doubt that you actually need to. The screws are primarily going to hold the planks in place while the glue sets.



Concerning the weight: You're adding 1.5 lbs. per square foot.

You're adding about 12.2 cubic feet of wood to the ceiling (15'×13'×0.75"). You say it's barnwood, so I would assume something like western cedar. That's 23 lbs/ft­³ (plug in a different value from here if it's not right). So you're adding a total about 280 lbs of wood plus screws/glue. Say 300 lbs. It sounds like a lot, but it's spread over the entire ceiling which is 195 square feet. That's about 1.5 lbs. per square foot, which is approximately the same weight as the drywall itself. I'd be surprised if doubling the drywall was enough to make the ceiling collapse, but I'm an electrical engineer, not an architect; so I could easily be wrong about that. Do check with a local contractor/inspector if you're concerned about it.