Diabetics have ways to estimate it using devices, but I believe a blood test is the only really accurate one.
I seen a number of 20% accuracy for home tests, higher accuracy when glucose is higher and lower accuracy when glucose is lower.
What do you mean?
That you cannot trust in those tests?
The "prickless" glucose testing monitors that I've seen use infrared light that are absorbed into the sugar molecules. The level of absorption is what the device measures and is able to give a reasonably accurate measurement, but it's not going to be as accurate as directly testing via blood. At least, not with the current level of technology. Maybe in the future they'll be able to improve upon it though.
If you're diabetic, blood glucose testing is still the gold standard.