There are no real ways to know for sure how many people go to your site from bookmarks, but a couple of ways

• (the usual) take a high %  (or even 100%) of unique WITHOUT any referrer data (that also are not a bot)
• Make a “bookmark this” link, but make the bookmarked url something like “http://example.com/?camefrom=bookmark”. Then just do something like

• if ($_GET['camefrom'] == “bookmark”)
  {
  // +1 to your stats thing counting bookmarks (database? etc)
  }