How can I find ceiling joists underneath this wood paneling?
That looks similar to T-111 siding that should be nailed to the joists every 16 or 24 inches. The key would be to find the seams where the pieces join. The other seams are just for looks and won't have any joists under them.

Using a thin piece plywood or shim to bang on, you can hold that against the ceiling and bang with a hammer to hear the hollow and solid spots without damaging your ceiling.

Good luck!