Personally I've never had much luck with electronic stud finders.
Other techniques that work for me:
- Use a magnet out of an old hard drive to locate nails/screws. Find one then search up and down for others. Hits in the top/bottom plate can be misleading. Wrap the magnet in some tape to help it slide over the wall without marking. I tried dangling the magnet from a string too, but that wasn't much help.
- If the wall is sanded/painted, use a very bright light at an acute angle to look for subtle imperfections in the finish that indicate a nail or a vertical joint.
- Get into the roof space and look at the top plate. Big nail heads can show where the studs are. This doesn't work for metal-framing, nor from underneath a floor.
- Existing fixtures - most power socket plates are screwed immediately to the left/right of a stud. So take a plate off the wall and have a look inside. You can also use a thin poker like a steel ruler or wire to slip behind the wall cladding and feel for the stud. (mind the power etc)
It is tempting to find one stud (or start beside a door/window) and then measure off 24"/600mm centers, or 18"/450mm centers, or whatever your home was built to. And that helps give a starting point for a magnet search. But there are many standards, and the spacing may vary across the wall.
For complete confidence, you can identify the center, create an access hole in the drywall, and mount in some wooden support dwangs/nods/noggins exactly where you need them. Of course this is the most work but the most long-term reliable solution. Additionally, this helps with hiding data/signal/aerial cables to have ready access into the wall. You might put a removable panel on the wall, or a cupbaord door, or drywall/plaster/paint the hole behind the TV.
Perhaps in the future you have a hundred inch TV which needs more support, so future proofing is a good idea. There is no overkill, especially if you live in an earthquake/tornado/etc region.