Read the listing or description. Or call the listing agent.
It's generally unusual for an annual rent to be listed. Usually, you'll see either a monthly rent or a seasonal rent. It's what (and in what amount) the owner expects to be paid.
Also, you often can figure it out if you have a sense of the area, or by looking at other somewhat similar rentals. Example: In 99% of the cases, a number of $900 or $1,200 is a monthly rent. In most cases--except for really, really exclusive areas or well-known high price areas, a figure of $4,000 or $5,000 is seasonal rent. (Sure, I can show you houses in McLean, Virginia, or properties in New York City that go for $5,000 a month. But you and I both know that those are very expensive areas.)
Hope that helps.