The Seller
Is the seller of the property an estate agent, or the owner, or selling it for someone else?
It may be that the owner has appointed a friend, a neighbour, a lawyer or estate agent to answer inquiries about the property. You should at least know who you are dealing with.
Ownership
How is the property owned?
Foreigners in Thailand can own buildings, but not land. If the property is a condominium, this will not apply, but if the property is a land plot, a house or a villa and the owner is a foreigner, the land will either be leased or registered to a Limited Company of which the owner is the director. Leases can be re-negotiated and Limited Company shares can be transferred.
Infrastructure
Ask for information about road access, electricity supply and water supply. Ask if there's enough water all year round, and if storm-water drainage is an issue in the rainy season. Ask about connections for telephone and internet, and cable or satellite TV.
Running Costs
It is not uncommon for property owners to pay into a communal fund for things like garden maintenance, pool cleaning, garbage removal and security. Ask about the cost of these services and how they are arranged. Ask also about the average cost of electricity, water and other domestic bills.
Documents
Make sure that the seller has photocopies of all neccessary documents to give you when you visit the property. These include the Title Deed ("Cha-note" in Thai), the seller's ID card or passport and the current lease contract or Limited Company papers. Your lawyer will need these documents to research the property.