Easy sufficiant service check?

Let's say I want to add a couple circuits for a garage or addition. I know the calculating method where you multiply sq. ft. By 3, add 1500 for appliance and laundry circuits, other fixtures... 10000 watts at 100%, and so on (the formula is not my question). Is there a way to get a general idea with a clamp meter. For example, if each leg of a 100 amp service is well under 100 with appliances, etc. turned on, is that a reasonable way to see if existing service is able to handle new circuit(s)? If so, would it follow the 80% safe capacity rule as with branch circuits?