I've just started using Freestyle Libre[1]. It's quite affordable even though it's not as accurate as blood measurements. It's possible to have one sensor attached for up to two weeks. This should be enough time to assess your body's reaction to carbs.
I don't know how you would test for pre-diabetes but to test for diabetes you can basically just drink a lot of sugar-water and see what your blood does. There are levels you can't reach unless you're diabetic.
Personally I've switched to a mostly carb-free diet because managing sugar levels is much easier that way.
[1] http://www.diabetes.co.uk/blood-glucose-meters/abbott-freest...