Let's say I have a slave flash (or strobe) in my home. I don't have (or I don't want to use) a sync cable to connect it to my camera. The slave flash will be trigger by the the built-in flash. The output of the slave flash is fixed, i.e. it cannot communicate with the camera to adjust its output.

In such scenario, do I have to use M mode? Does the auto-exposure still work? And how?