I suspect a lot comes from friends in the know, or contacts within the industry. Some people may ask at checkin or steal a glance at an ops computer (or fly through FRA where you can get the ops info from passenger info terminals).
Also, some spotter groups (such as 'manchesterspotters2' on Yahoo) have people involved who may provide such information on request; or can provide an expected aircraft based on observed operational patterns; online services such as flightradar24 can be an aid to this.
In short, lots of ways and no single answer!