In the benchmark system information pages, if it says 'villec2' anywhere then you have the S3 version. If you have the S4 version it might say 'ville' or 'MSM8960' in the case of Vellamo.
Also the firmware version series are different, the S4 one had the initial firmware starting with 1.53 and now has 1.78, possibly 1.84, 2.21 and 2.23 updates. The S3 version has firmware revisions starting with 1.11.
Finally, if you start the phone in bootloader mode, if it says VILLEC2 you have the S3 version, if it says only VILLE you have the S4 version.