Difficulty finding studs on external wall of renovated house - any good rules of thumb?
Since it's an exterior wall, you may have &quot;cross strapping&quot;. It's a technique to frame a wall using 2x4 studs, and then cross strapping it (horizontally) with 2x1 straps to increase the space for insulation.
In this case the drywall is fastened to the strapping, and the fasteners will be aligned horizontally, rather than vertically along studs (see drawing).
They could run 24in or 16in apart (vertically separated)

From https://www.nrcan.gc.ca/energy-efficiency/energy-efficiency-homes/make-your-home-more-energy-efficient/keeping-heat/keeping-heat-chapter-7-insulating-walls/15641
If this is your situation, you would likely not be able to use the straps for hanging curtains: there will likely be no strap where you need one, and they are not for bearing a load.
The solution is to use longer (2.75 - 3in) and stronger screws (#10) to reach deeper to an underlying stud or header.
The best bet for locating the studs is at the edges of the window opening. That's where you'll find the &quot;king&quot; and &quot;jack&quot; studs (see drawing).

From https://www.nrcan.gc.ca/energy-efficiency/energy-efficiency-homes/make-your-home-more-energy-efficient/keeping-heat/keeping-heat-chapter-7-insulating-walls/15641
The stud finding techniques in the other answer, by Jon, are indeed good, but with strapping it's still hard to get reliable results from either.
Also, a stud finder with &quot;deep scan&quot; might or might not work. I've had no success with one.
If you are ok with a bit of drywall filling, you could carefully drill a 0.25in hole through the drywall and poke through it with a 3in nail, coat hanger etc.. to see if you hit a stud after the strapping space of about 0.75in. There will be insulation to contend with: and becareful not to twist out insulation or vapour barrier that gets caught on the drill.
If you are worried about punctures in the vapour barrier after all the drilling, you can squirt in some expanding closed cell foam through each hole, to re-seal them, before you patch the hole.