The normal check for the standby power is during the preflight cockpit prepartion. just as it says in the Boeing manual.
The check assumes that the Aircraft is powered by an A/C power source which is fed to the Essential A/C buss and via the T/R to the Essential D/C busses.
If the Battery master switch is placed to On and the Standby Power Switch is in Normal or Manual the Standby busses will be powered and the correct indication will be shown on the N1 and EGT indicators. All powered, all normal.
Howerver, if you want to induce a failure, ie outside of the system test requirements.
Either,
1. Prior to switching on the External A/C power or APU A/C power, check that the Standby Power Green ON light illuminates when the Battery Switch is place to ON and the Standby Power Switch is placed to Normal or Manual ON.
or
2. If A/C power is esstablished to the Essential A/C and hence to Essential DC busses via the Essential T/R, either by the engine or APU generator/s or External Power, then, if you have to, select the Essential A/C Buss Switch to OFF and Essential DC Bus Isolation Swtch to Open. This will induce a failure of both the Essential A/C and Essential DC buss bars.
However, this is not something that you would want to do in normal operations. Leave this to the simulator or the maintenance department. Nice to know, not nice to do.