How to find ceiling joists underneath this wood paneling

The studs will likely be at 16" intervals from a corner, but that corner could be the corner of the building and not the room if you have a wall T'd with another wall. The first 16" could also be more like 15" depending on how the walls are laid out. There are several places you can check to give you more hints:

• The first, and easiest is any outlets and switches. They should be attached to a stud in new construction.
• The next is any trim around the floor or molding around the ceiling, which should be nailed at the studs. You may be able to see small bumps from any nail filler on these.
• And the last method I can think of is to use a light at a sharp angle to see if you can notice bulges in the drywall at 8' intervals. Those are the non-beveled edges at the end of the drywall, and you should be able to notice them if the drywall was installed horizontally (frequently done, but not always).

Beyond those non-destructive options, and the stud finder that you've tried using, you're left with drilling test holes. I'd suggest making the holes close to the floor where they will be less obvious, but not so close that you're hitting the base plate of the wall (you need to be about 2" up).

You should shim under the loose joists with steel shims, not cement, because cement causes wood to deteriorate. Large galvanized washers and small steel plates can be used as shims. The framing block in between the joists are called bridging and in an old house I would space them about 4' on center. You can off set each row by 1 1/2" which will enable you to blind nail through the joists which minimizes toe nailing. Use 3/4 tongue. And groove ply glued and screwed to the joists and bridging. I would use pl400 for glue and 2" exterior screws spaced about 8" apart. If a second layer of ply is desired you should also glue and screw it the same as the first sheet. Try to hit the joists. Use a chalk line to mark the joists. If a solid wood finished floor is being installed 1 layer of 3/4 plywood is all you need.if a ceramic tile,stone or wood laminate floor is going in use 1/2 ac plywood over the sub-floor.