How can I know the 'capacity' of an electric socket?
If you have a device that draws 10amps, then the entire circuit needs to support that draw. This includes the circuit breaker, wire gauge, length and receptacle rating.  

In the US where the household voltage is 120V, a typical lighting or outlet circuit is 15Amps, but you're only supposed to use 80%, so that leaves 12amps available. All other devices on the circuit count towards this limit.

So if your device needs 10amp, that means you can only have an additional 2 on the circuit.

Watts = Volts * Amps

240 Watts = 120V * 2amps

So as long as the wattage of other devices on the circuit add up to 240 or less, you would have enough capacity.