Home Improvement Questions? Ask a Handyman for Answers
Welcome to Just Answer, my name is ***** ***** I will do my best to help you with your issue. If my initial response doesn’t answer your question then let me know and we can continue our conversation.
Your best bet is probably to get a stud finder. They are pretty good at finding concealed studs. http://www.homedepot.com/p/Zircon-StudSensor-HD55-Stud-Finder-65945/205143494
You mentioned magnets. This type of stud finder doesn't use magnets. I'm not clear on why you're looking for studs when it sounds like you want to open up a wall and install a door.
Well, if you can't find the studs with a stud finder then you'll have to remove a section of sheet rock where you suspect the old door opening was. Worst case you'll have a bit more drywall to patch.
I never said to remove complete sheets only to cut a hole in the wall where you think the opening was and work out from there.