Hi Gordon The best way I have found to find studs behind plaster is with an electronic stud finder. This little device emits a sound wave that bounces back when it strikes something of higher density, and the good ones have a 'thick wall' feature that essentially boosts the sound wave.....but has a higher liklihood of giving false readings. Once you find a stud, other studs should be 16" or 24" on center from this point. These stud finders are available from HD or most home improvement centers and cost $16 or so.
Lath is a poor base for securing wall hangings....unless the hangings are very light and you use a screw (nails will be almost impossible to drive into lath and will only succeed in knocking the 'keys' (or plaster that sticks between the lath and thus holds it in place) out. Heavy things like book shelves, TV's, large wall hangings, etc MUST be mounted on studs.