Firstly, we can use the equation V = IR to calculate voltage across a component when we know it's resistance and the current flowing through it, where V is voltage, I is current in amps, and R is resistance in ohms. In it's current state the equation is probably of little use to the mobile DJ, but we can rearrange it and come up with the following equations:

R = V / I
I = V / R

If you have a multimeter you can quite easily measure the resistance across a fixture, and you already know the mains supply voltage, allowing you to calculate the current consumption of the fixture. However, you would have to make the assumption that the resistance of the fixture always stays the same, and this is simply not true. For example, a par can gets very hot when running, and the resistance of the bulb filament increases as it heats up. Transformers and other circuitry inside intelligent lighting would also cause problems when trying to measure resistance.

Another useful equation is P = IV, again I is current in amps, V is voltage, and P is power in watts. This equation is probably more useful than V = IR, as in many cases you will know either the wattage of the fixture and want to calculate current, or vice versa. Again we can rearrange this equation:

I = P / V
V = P / I

There are, of course, other equations, but the ones I have detailed here should be enough for the average mobile DJ. Any lighting or sound technician that doesn't know these equations probably needs to go back to school!