The correct way to ensure your lights are set to the correct strength is to use the waveform monitor, which is what any videographer, photographer, video editor and colorist would use.
In 2.8, you can find it in the Image Editor or the rendered view (F12), Sidebar (N), Scopes, Waveform.
It displays the luminance of your image, left to right as a graph.
Use the Luma setting. The highest peak should be under 100, say between 80-100 unless you intentionally want an underexposed image say for a night time or cave scene. Generally you don't want to underexpose. If your output is going to be broadcast (e.g. a motion graphics commercial on TV), your waveform should peak under 90.
You usually expose your camera for the key light as that is the most important, but keep an eye on the exposure for the whole scene.
To brighten your image you can either increase the strength of your lights individually or increase the exposure on your camera (Render, Color Management). Also, you should be using the Filmic View Transform (default in 2.8) found in the same place. This will make it very difficult to overexpose your image, among other very good things.
You should NOT try to visually guess if your scene is properly exposed unless your monitor is properly calibrated, and you've trained your eye with one of the disciplines mentioned above or something similar like 3D production.
