When looking at the receptacle with the ground plug on the top:
A 120 Volt - 20 Amp receptacle has a vertical slot on the left and a "sideways tee" slot on the right. That receptacle will accept either a 20 Amp plug or a 15 Amp plug.
It is permissable to use the more common 15 Amp receptacle (both slots vertical) on a 20 Amp circuit.
A 240 Volt - 20 Amp receptacle has a vertical slot on the RIGHT and a horizontal slot on the LEFT. Some (example in link below) may have a tee slot on the right to accept a 15 Amp plug.