The one dead giveaway is big cracks. That being said, it’s important to remember that not all cracks are cause for panic.
Most buildings get small cracks as they settle under their own weight. Fine cracks in newly plastered walls are not unusual, and minor cracks where walls and ceilings meet can be caused by changing temperatures.
The real signs of a subsidence property are diagonal cracks which suddenly appear in plaster work inside and the bricks outside.
This will normally happen at weak points like around doors and windows, especially after the weather’s been dry for a long time. The cracks are normally wider at the top.
Cracks at the join where an extension meets the main building can mean it’s pulling away from the house, and doors and windows may also have difficulty opening and closing due to the distortion of the building.