Simple blood tests will identify this condition. Pre-diabetes can be diagnosed if the fasting blood glucose is slightly high (6.1 to 6.9 mmol/l by WHO criteria).
There has been a lot of discussion on the actual level at which fasting glucose becomes abnormal, for example even a level of 5.6 mmol/l is considered abnormal by American Diabetes Association criteria. This condition is called impaired fasting glucose (IFG).
Alternatively a diagnosis of pre-diabetes can be made following a glucose tolerance test where after a standard glucose load the 2 hour blood glucose remains mildly raised ( 7.8 mmol/l to 11.1 mmol/l). This condition is called impaired glucose tolerance (IGT).