Well by tasting it and feeling that it is too sweet for a savoury dish. And you have the feeling that it misses something, but it is salty enough. You have to train your senses about the sweet stuff. If you are used to adding sugar to your savoury dishes, then it might be difficult to spot the sweetness from a vegetable. Now, not everything needs acidity. I would not make a chicken soup with noodles an acidic one. But I do add a spoon of apple cider vinegar at the beginning of cooking the chicken, since they say this will extract more minerals from it. But it is not mandatory. But if I make a curry for example, the sweetness of the coconut milk is too much, so I add either some lemon juice, or some tamarind paste. Little by little, until it tastes balanced.

Sometimes it happens with salt, I taste the dish and I feel like it misses something, but not acidity or sweetness l, so I add a bit of salt and see if it improved. With time, you kind of know how much to add. Another trick regarding salt, you can salt all the ingredients separately. Since the quantity is smaller, you can visually tell how much you need, as if when you salt something on your plate, or the salad you just made.