Your house has studs onto which lath board is nailed. Lath is thin rough wood that is attached to the studs and is about an inch or two tall. It is spaced a small distance from the next lath board so that a slit is formed between the rows. Plaster is then applied in usually 2 coats - a rough coat that adheres to the lath and a finish coat that is used to float out the rough coat and creates that smooth wall.
You can look for studs, but I have found the best way to hang heavy objects on the plaster wall to be these hooks made especially for this purpose. Your mega home center (HD, Lowes) has them in their fasteners aisle. It is a picture hook called UGL or something like that in a blue and yellow package. They are rated at 5 pounds to 250 pounds depending on the size. You lightly tap the needle like nail into the wall through the hanger. It works like a charm.
In my parents' condo in DC, we attached a 125 pound mirror to the wall above their bed with these hooks. We are that confident in these hooks.
I have lived in homes with plaster all my life and have yet to find an easier way to hang things.
You can, however, use other types of hangers specifically made for plaster walls. I've found toggles to be a pain because of the lath boards and other methods to unkey the plaster from the lath - not a good thing.
I've had a lot of experience with plaster repair and work over the years and would be happy to answer any questions you may have.