For most homes for sale on Trulia, you are able to contact the listing agent (agent representing the home sellers) by clicking on View Details or Contact Agent buttons.
If you would prefer to work with an agent representing you, this could be any other local agent under a Buyer's Agency Agreement. This form partially explains who an agent works for in a real estate transaction. http://detroitassociationofrealtors.com/forms/FORM%20K.pdf
Good luck in your search for a home. If you need assistance feel free to contact a Trulia member.