I was able to find floor joists through 3/4 oak+5/8 plywood with one of the better 'deep-scan' stud finders. As I recall it was about $40.
Not 100% reliable, I mark positive readings with tape until I get readings 16" appart.
Good 'flir' type camera may get readings also.
As an aside, it took me way too long (and many burned out masonry and tile bits) to realize that these are the best bits for drilling tile & Porcelain:
https://www.homedepot.com/p/Milwaukee-3-16-in-Diamond-Plus-Hole-Saw-W-Arbor-49-56-0503/204994399