What is the difference between semen and sweat around the penis, and is ghusl obligatory if wetness is found that is suspected to be semen?

Semen has known characteristics that distinguish it from sweat and other substances. If you are certain or strongly suspect that what you found is not semen, then ghusl for janabah is not obligatory upon you. If you are uncertain whether the discharge mentioned is semen or something else, you have a choice: if you wish, you may consider it semen and perform ghusl, or if you wish, you may consider it madhiy (pre-seminal fluid) and wash only what it has affected.