Monitor/see session by IP

There isn't anywhere to view actual session info per se but you might find the info under "Policy > Monitor" to be helpful.  Creating an extra policy to isolate the traffic you are interested in to get a more focused look at the info you are needing will help if necessary.  Otherwise you can configure your policy for logging all sessions and view the traffic logs.  

Those are probably your best options unless someone has another alternative? 

In FortiOS v5.2, use the FortiView menu entry. At first, it looks like a session table but you can filter and drill down into the sessions if you need. This replaces the Policy Monitor and the session widget with a much more powerful alternative. See the Release Notes for v5.2.3 and v5.2.4 for latest enhancements.