Quote Originally Posted by Pieter
If the flash mounts on the hotshoe, it should fire and you can use it.

Do you mean in general, or in the specific case of the Speedlite 188A? The Speedlite 188A is safe to use in a current Canon dSLR/G-series hotshoe. But current Canon cameras need triggering voltages of 6 V or less, and some older flashes have much higher triggering voltages, meaning they can damage modern cameras. See this complied list of flash triggering voltages.