Generally speaking when considering the purchase of a house (and or land) you need to review your proposed purchase with regard three principle matters. 1) The ownership and legal status of the title / property, 2) The condition and likely future status of the surrounding area, including access and utilities availability and 3) The physical condition of the built structures on the property.
Since your inquiry seems to focus mainly on the due diligence related to the physical condition of the house, I will only deal briefly with the ownership and legal status issues ( which have been covered extensively in earlier articles) and the local area issues. Unless you have a lot of prior experience in this field and read and write Thai, I would always suggest that you take on the services of a lawyer or reputable property consultant to check ownership, title, building permits, utility supplies and development plans in the surrounding areas that might have future impact on your proposed new property.
Building surveys are an almost standard part of the due diligence conducted when buying a property in most advanced western countries, and a whole profession of chartered surveyors has been developed to provide this service and issue meaningful certification of their results. Sadly there is no such equivalent profession or established recognized professional body in Thailand, and Thai purchasers (or vendors) are not typically conversant with the concept of a building survey.
The general procedure used by Thais and indeed most foreigners purchasing a domestic sized building in Phuket, is to take a good close look at the building themselves (or perhaps ask a friend with more experience) and then apply a little common sense to the process.
What should you look for? Primarily this should be water leakage issues and structural (or foundation settlement) issues. These defects are generally easily discernable by cracks or stained paint work ( or obvious recent repairs of the same). Take a look in roof spaces, cupboards under sinks, out of site spaces at the back of a building. If all looks clean well maintained and in good order, you are off to a pretty good indication of a sound property.
If all does not look in good order, or you just want a second opinion, you do have a few options. Many of the professions allied to building, be it architects, engineers, project managers or even contractors, will generally be well placed to give you a good professional ( albeit not certified) opinion on the state of a building - and indeed many will even deliver a written report in very much the same format as would be obtained from a chartered surveyor. These people can also (if required) budget the likely costs of putting a building back into good order or effecting any additional improvements to the house that you might be considering.
As to the question of who you can trust, that is really a matter of seeking some recommendations -Your local real estate agent or lawyer , will usually have worked with several firms who have provided this service in the past and will generally be well placed to give you the required recommendations based upon past experience. By PhuketLand.