Finding ceiling joists in an old apartment
If the hole you drilled already is larger than a quarter inch, get a cheap usb borescope inspection camera and stick it in there to take a look.

They're available for phone and laptop connection on amazon for under $20 USD.

You can also use a FLIR camera to find studs pretty easy.
 

FLIR cameras are a bit more than $20.  Think $2000 for good ones.  But you can get lite versions for android and apple smartphones for &lt; $300 USD and you can usually rent them the advanced ones at your local home improvement stores for less than that.

TIP:  When you rent a flir camera, use it to photograph EVERYTHING in your house.  EVERYTHING.  Save the pictures for future reference.