Is it possible to detect a load in an a.c. circuit before switching the main power supply supply on? Perhaps by using an auxiliary circuit which has it's own power supply, like a multimeter?
There is no generic way to do this. However, if you have some knowledge about the type of circuit to expect as load, there is.
Simplest problem is, when you can be sure that you will find an ohmic load. Then you can probe it with a small voltage and measure the current.
An ohmic but not linear load like a incandescent bulb is also possible, but you have to probe it with sufficient power to gain some points (3) to get a usable U-I curve.
If you assume purely inductive or capacitive loads you can probe with a.c. When you want to know about series resistances you can probe with d.c., too to gain some characteristics.
In general be aware, that probing for big loads surely needs some power to get reasonable results which aren't completely derailed by effects like high power semiconductors which refuse to work at lower voltages.
It is nearly impossible to probe for something like a SMPS in my opinion, due to the manifold architectures of the input stages.