To find out the listing agent or listing brokerage on a property, you can contact a local realtor to find out via the MLS. Depending on where you found the listing, it most likely will have the listing agent's information attached to it. You can also search for the listing on Trulia and find out this information.
Depending on how the information was delivered (flyer, email, online post) either the listing agent will be attached to it or at least the listing agent's firm. If not, the MLS # will be the best way for your agent to quickly and easily find the listing information.
If you need further assistance you can contact me directly.
You would get this information from the MLS which is available from either an agent our clinging the respective association where the property is located. If it was not listed on the MLS then it is a little more tricky.
You can give me a call I'd you need more information.