There are some symptoms of diabetes that you can look for, but you won't know for sure until you see the doctor:

fatigue

frequent urination

increased thirst

increased hunger with possible weight loss

and if you really want to you could drink some of your urine to see if it tastes sweet

Of course these are very general symptoms that could be explained by any number of other things.

(Since someone recommended the home glucose testing, this is an option, but keep in mind there is up to a 20% error rate. What you should look for is a fasting blood sugar of under 126 and two hours after eating it should be less than 140. Home glucose tests are not used to diagnose diabetes because they can be so inaccurate, so take caution with the results you get if you go this route.)