From my H2S training:
TOXIC AT CONCENTRATIONS ABOVE 10 PARTS PER MILLION, H2S GIVES LITTLE WARNING SINCE OLFACTORY SENSORS ARE IMMEDIATELY DESENSITIZED AT LETHAL CONCENTRATIONS.
By the time you smell it, you are already dead.
We also use H2S detectors. The problem is, you are wearing it. By the time the sensor alarm goes off, chances are, you already inhaled it too. H2S sensors on the perimeter would work, unless the source is at your work site.
Unfortunately, and this probably happens way too frequently, and is kind of morbid, but, a common indication of H2S present is your "buddie" going down.