To test the flash with an incident meter you would need to point it at a gray card. But there is a catch if you point it at the gray card where can you put the meter? So may be a large gray wall and with a small meter in the center it won't affect the reading much? The readings are likely to be lower because I read Metz publication somewhere that their flash expect a subject of 50% reflectance and not 18%. In reflected mode, you can point it at a blank wall and expect the reading to be close. Again I found most flashes give lower readings.
To test the flash in manual, all you check is their power and not how well their auto mode operates.