To a certain level you might not feel if your blood sugar is high or normal. But after 250 mg/dl, especially at 400 mg/dl you'll feel it. The most obvious symptom is fatigue and thirst, no matter how much you sleep you will feel exhausted all the time. You will want to drink a lot of water, you will urinate frequently. Your eyes will start to hurt. After being high for some time, your vision might get blurry, your eyes dry, you'll start vomiting.

Also you could smell your urine, if it stinks like ammonia, your blood sugar levels are high.

If your blood sugar levels are normal you feel healthy.

I've been a T1 since I was 10 and this is my experience. The real problem is not noticing highs but noticing lows. Because low blood sugar is much more threatening than highs for the short term.