A single stage argon, oxygen, or nitrogen regulator is normally somewhat smaller than a two stage reg. There is usually a giveaway bulge in the back for the high pressure first stage on the two stager, which the single stage one doesn't have.
These are CO2 regs, but the identifiying features are common to those for other gases. Both images are of the rear of the regulators.
The left hand reg in the first image is a single stage, right hand reg is two stage. The high pressure first stage is clearly visible in the right hand image.