You can't really. You can ask, but an agent won't tell you. They'll tell you if you're too low, maybe, but that information is not given out by the listing agent because it probably wouldn't be in the best interest of the seller.

Jessi,
I'm sure you can understand why that information is kept confidential. You wouldn't want your agent sharing that info about the house you were buying. And, for the sellers, in case the deal falls apart, they don't want to tip their hand about what they would accept when entering in to new negotiations.

As Aaron says, once it closes, that information will be on public record.

Once it closes the sale will be public on tax records and available to your agent via the MLS. Prior to that it is almost always kept strictly confidential.