When I sauté foods in ghee, even over just medium heat, something like smoke starts coming up. Is there any way to check?
Smell it - smoke will smell, well, smoky! Steam is liquid evaporating so it will also be wet whereas smoke will be dry.
Ghee is pure fat - there is no liquid, so if it's coming from the ghee, it must be smoke.
The food itself however, would be giving off steam as the water in the food evaporates.
So the question really is, "Is it coming from the ghee or the food?".
The ghee itself should not have any water, that is the point of clarifying the butter. So if you are melting ghee in a dry pan and vapor come up, it is probably the ghee burning. However, if you are cooking vegetables in ghee, they WILL have water content that may be driven off as steam. A tip from my environmental air quality engineer husband -- steam is white, smoke is... not. Food steam smell like what is being cooked, but more delicious and savory. Smoke smells of what is being burned.
Ghee has a fairly high smoke point. I can't imagine it would burn on medium unless it wasn't made properly and the milk solids weren't removed. That is what would burn, but you would see it. You aren't mentioning burning smell or burnt taste, so I think it's just evaporation of whatever food you are cooking.
Water turns into steam; that's what you'll see what you heat up vegetables or anything with water in it that turns into steam. Steam has a sort of clear appearance and it doesn't have a scent (except that it often "carries" the smell of the food up with it).
Oil - such as vegetable oil, butter (containing oil and water), or ghee, will start to shimmer as your pan heats up, and then it will start to smoke. Smoke smells smoky :)
Looking for your oil to shimmer and then to smoke is a great way to tell how hot your pan is, and therefore to know when to add your food to the pan. Some foods are best added to the oil /ghee when the oil shimmers or the ghee melts, and some foods are best added when the oil is just beginning to smoke, that is, when you first see those whisps appear.
Back to your question, you'll know if it is smoke or steam based on what is in the pan. If it's oil/ghee/butter, it will produce smoke (I would not want my butter to smoke, that's burnt! - but I'm not as familiar with ghee). If it's food like veggies that contain water, it could be steam.
Just read a bit more, and since ghee is clarified butter, then it would only produce smoke and that would be fine to cook with for certain recipes where you want the pan really really hot.