Body level is all about the perception you get from the mouthfeel: does the wine feel thick, with a mouth-coating density?
If in doubt, let the looks of the wine give you hints, but it’s really by tasting that you can nail it.
Body level has nothing to do with the wine’s quality. Some ‘experts’ will claim that the best wine brings everything - body, oaking, ageing, etc - to the fullest.
In reality, great wines are usually a balanced blend of all of these or even simple purity of one of the elements over the others.
Body in Reds
Most red wines are medium or full-bodied, but the examples of great light-bodied reds are many. The looks of the wine can help a lot in getting body levels right with reds, and then tasting will confirm the final answer.
Body in Whites
Nailing body levels with whites is tricky. Their colour might not help you as much as with reds. They may seem bound to be light-bodied but that’s not always the case. Mouthfeel and consistency will guide to the right answer.