question

The only way to ascertain the surplus available power is to fully load the array to find out the maximum that can be produced and then subtract that from whatever the active load is.
Just like you suggest, a quick load needs to be applied to find out otherwise there is no way to know what the array is capable of producing at that moment in time.
If the battery is full and the loads are satisfied then there is no other load to apply to the system to perform any calculations.Otherwise it's just a complete guess which will most likely be wrong.
This is how those surplus PV water heater gadgets work. They use the heater element to load up the inverter until either the battery starts discharging or the system starts to take energy from the grid and then back off a bit. This feedback loop is monitored continuously as the PV power is constantly variable too.

The accepted way of doing this is to add a calibrated irradience sensor to the installation, this will give you an indication of the current w/m^2 (this can be converted to kW/kWp by the area and efficiency of your array). This can also be integrated to give the cumulative kWh/m (or kWh/kW) per day. This can be compared with your actual consumption to give the excess. Whilst not everyone wants to spend the $800 or so on a calibrated sensor, a home made version can be made from a small solar panel working into a dump load. The power into the dump load can then be monitored, and calibrated against your existing array whilst that is on full load (i.e. in bulk charge).