My little one was diagnosed as a type 1 diabetic only 6 weeks ago (she is 2 yrs 9 months old)
There are many symptoms to look out for including constant urinating, frequently drinking (my daughter was drinking upwards of 6 litres a day), being lethargic etc.
My daughters BSL should be between 5 and 12, and adults are between 3 and 8. So I would be concerned with anything above these ranges.
When my little one was diagnosed her BSL was above 30. The thing to remember is try to do a sugar check 2 hours after she last ate (as this is when the carbs/sugars are usually broken down and processed)