Investigate the area
Spend some time walking around the area, observing things such as proximity to shops and cafes, transport links, and noise. Be sure to revisit at different times of the day, particularly rush hour, to see how light and traffic changes. You don't want to find out after you move in that the quiet street at 10am becomes a clogged road at 6pm!
Look at the structure of the building
Make sure that you check the home for damp, giveaway signs are a mouldy smell, peeling paint and watermarked walls or ceilings. In addition, hairline cracks could be a warning sign of structural problems. Replacing a roof can be incredibly expensive so do also ask/check for missing or loose tiles or broken guttering. Finally windows, if you can easily push your finger into a wooden window frame, it is likely to be rotten, even if it's been cleverly given a coat of fresh paint!
Check, check, check
It's also important to check internal fixtures such as taps and light switches. How is the water pressure? Does it take long for hot water to come through? Don't be carried away by clever staging, strategic lighting, fresh paint etc. It's important to check the boring stuff too!
There is no such thing as too many questions
Prepare a list of things that you want to ask, as it's easy for things to slip your mind once you're inside. Some things you may want to consider putting on your list - ask what fixtures are included