Start with the tap test. Using your knuckle, tap wall and note difference in sound between hollow areas and areas that are less hollow. It is likely at centre of least hollow pitch is a stud. Crosscheck this by seeing if magnet can find a nail or screw in wall, either above or below where you found stud – try about 1200mm above floor as that is where edge of plasterboard sheet is often nailed.
Leave magnet in place, then double-check using electronic stud finder. On stud setting, it should be green away from stud and it will go orange when you are approaching stud.
When finder turns red, it's over the stud. A further check is whether there is another stud 450 or 600mm to each side as this is the normal stud spacing, depending on age of house. Other telltale signs for stud positions are protruding nail heads and power or light switches, which are often secured to a plate that in turn is fixed to a stud. Check for any metal plumbing or wiring.
Now, it depends on what you are hanging. If over 10kg, it’s best to fix to a stud, but otherwise you can use a WallMate or other hollow wall anchor. When using WallMates, use a hand screwdriver as you want the fitting firm but not too tight, or you may strip out plasterboard. Drive pan-head screw in centre hole, then hang your artwork.