If it is existing, then all you really need to do is measure amp draw with everything on. Only id this amount is more than 80% of the main service size do you need to look at it closer.
As for individual circuits, look at the FLA of the equipment it's feeding, multiply by 125%. The circuit ought to be that big, or larger.
For the sub-panels, treat them like the main service: measure at full load, see if it's large enough.
