How do I tell if I have diabetes?

Go see your doctor. Assuming you are not pregnant, there are 4 ways to tell if you have diabetes. If you have classic symptoms such as increased hunger, increased thirst, increased urination, or blurred vision with random blood glucose =/>200, 2 hour 75 gram glucose tolerance test =/>200, fasting blood glucose =/>126 (x2) or hemoglobin A1c =/>6.5. Please go see your doctor to evaluate you.
Blood test. A fasting glucose is a good start a 2 hour glucose tolerance test is recommended if the fasting is normal but you have risk factors for diabetes like obesity or a family history.
Excess thirst. Excessive thirst, frequent urination are usually early symptoms. If in doubt please get checked.
HbA1c, NMR, ↑AfterFood. Hba1c (reflects average blood sugar of last 1-3 months) best, optimal ≤5.0%; ≥6.5% is clear dm. Even better is NMR particle test insulin-resistance score (can detect ≥10 years prior to hba1c ≥6.5%). At home method, check peak blood glucose, 20-90 min after high carb meal, highest should be ≤140 mg/dl. But this must be done many times, is more complex ; less effective; more hit ; miss.