Display station and workstation are normally the same (unless you’re using the system/36 environment). You can normally find which user is using which workstation by using WRKACTJOB, but if you want to search by workstation, then you could try WRKCFGSTS, or if you want to look by user id then you could try WRKUSRJOB.
Or do you mean physically locate where that user is signed on? That can be quite difficult, especially when most users use PC’s to access the system. If you still have (green screen type) terminals, you can use PRTDEVADR to find out which port they’re attached to and which address they have, but with PC’s they could be anywhere and you often don’t even see which IP address they have.