Tracking a SMS source

In order to track a SMS it needs to contain a clickable link. You can attach a campaign tracker to the link, specifically a custom marketing campaign tag, that will enable you to separate visits from URLs containing the tracker into an "SMS" marketing source for example.
You will thus obtain all the visits that have the SMS as a source.

Otherwise, the SMS could contain a link to an entry page specific to this channel. You can then check the data for said entry page to look into the visits started via the SMS.