An agent at a viewing will answer what they are asked. Ask the right things and you learn more in fifteen minutes than in a week of emails. Some questions should be asked in writing afterwards, because a written answer that turns out wrong is a misrepresentation.
At the viewing
How long has it been on the market, and has a sale fallen through. Why is the seller moving. What is included: white goods, curtains, the shed. When was the boiler last serviced and how old is it. Where is the stop tap, the fuse board, the attic hatch. Is there a management company and what is the service charge. Has the house ever flooded. Is there a septic tank or is it on mains. Was it ever tested for radon. Any knotweed on or near the boundary. Has any work been done without planning permission. Are there any disputes with neighbours over boundaries, parking or trees.
In writing
Four to put in an email after: has the house or garden ever flooded and was a claim made; has there ever been a radon test and what was the result; is there or was there Japanese knotweed on the property; has any extension or conversion been done and was permission or a certificate of exemption obtained. Sellers answer these in the pre-contract enquiries anyway, but by then you have paid for a survey.
What you should already know
Before you go, the public record can tell you whether the house is in a flood extent, what the radon prediction is, its BER and the advisory report, what sold nearby and for how much, every planning application within 300 m, whether the title is registered and freehold, and what the sewer situation is. That is what the free snapshot flags and the full report lays out. Walk in knowing the answers and the questions become a test of the seller, not of the house.