the easiest to check is by look at the label (power rating) if there a 12v1, 12v2, etc.. it show that it has multiple rail (the number mentioned is the number of rail it had)... if it only 12v than it single rail....
To know if it really multiple/single rail it need to check inside of the PSU, u will need to search for review that do that... (some PSU fake their labels, usually on cheap unknown brand)
The Seasonic S12 II is a single rail power supply with a full 40 amps on the 12v rail providing 480 watts.
The website calls it a dual rail unit with 2 x 20 amp rails but this is incorrect. Read the Hardwaresecrets review.