Hi @Albert Chun - First, you should realize that unless you have a buyer's agent, that you are paying a commission on the sale, any agents involved in the sale are working for the seller. Granted, professional ethics should not allow them to actively deceive you but be prepared to verify any information you are given, either by your own research or by a financially disinterested third party.
That said, you should ask about any known physical problems : aging roof, damp basement, due-for-replacement HVAC system. I believe that most states have a disclosure form that an agent is supposed to fill out with the owner. Obviously, the value of this disclosure is in direct ratio to the honesty of the owner. But it is a start.
Next you should move on to the financial aspects of the transaction. You should ask about the amounts of the previous years property taxes and average utility bills. Verify that there are no special assessments known to the seller. Ask about any HOA involvement, and if so request a copy of the bylaws and a full schedule of fees.
If the answers to those questions are satisfactory and your own interest in the property has survived a visual inspection, spend a little time checking out the neighborhood. You can fix a lot of things, but you cannot fix a lousy neighbor.
If the house is still a candidate make an offer with sufficient earnest money to have it taken seriously. Use inspection contingencies to make sure that the seller and agents have been reasonably forthcoming as to the condition of the property.