* What kind of camera are you using
* What model of flash is this
* What sort of environment, e.g., studio, outdoor, general is the flash being used in
For some flashes, you can go into an "auto" mode that is not dependent upon any camera circuitry. In this case, there is a slider on the flash that based upon the speed of the film and distance, gives you an aperature to select. A thyristor sensor will then adjust the flash level given the amount of light needed.
The simplest manual method is a guide number. Each flash that has manual capability has a rated GN for full, half, or other incremental outputs. Again, this is a speed plus aperature plus ambient light level sort of thing. Both of these systems usually presume 1/60th of a second sync speed on the shutter.
The most precise way is to invest in a light meter that does flash as well. These range from used on fleabay to very expensive and new. I have a small Gossen DigiFlash meter that works very well, and is a reasonable cost.
something witty and profound needs to be inserted here...