Spend one full day casually observing your balcony at different times. You don’t need to sit there all day; just look at it in the morning, around midday, and late afternoon. Note when the sun actually hits the floor or railing, and how long it stays. If you get 4–6 hours of decent light, many sun-loving plants will be happy. If sunlight only brushes it briefly, you’re in low-light territory.
Another simple trick is to look at shadows. Sharp, dark shadows usually mean strong direct sun. Soft, fuzzy shadows suggest bright but indirect light. No shadows during the day usually means it’s too shaded for most flowering or edible plants, though some shade-loving varieties might still work.
If you’re unsure, start with a couple of hardy plants and see how they respond over a week or two. Plants are honest; they’ll tell you with their growth whether your balcony is a sunny beach or a quiet cave.