To find a stud use a strong magnet it will locate the nails in the stud that is holding the lath on the stud . If you are not hitting a stud us a toggler snaptoggles 3/16”. Do not over do it as that will make the wall weaker. If your hanging a tv bracket you only need 4 toggles at most you could even get away with 3. Each 3/16” toggle holds 238lbs per toggle in 1/2” sheet rock. Plaster and lath are stronger the 1/2” sheet rock. I would hope your not hanging anything close 238lbs on a wall.