Thanks to member Sjdjan for finding this map.
Expand map, zoom to areas and mouse over the bus icons. It will show bus, trolley bus and tram numbers that stop, here you can manage your route around the city.
Green = Bus
Orange = Trolley Bus
Yellow = Tram
The vehicles all have the numbers and final destination displayed, the stops on site show numbers and frequency, most are very regular. Do remember to swipe your card on the screens on entry, hold card over the screen until it registers and shows payment taken. Cards and credits can be obtained from any kiosks you see all over the city.
At peak times most vehicles get very crowded and can be uncomfortable, watch belongings, have wallets/money in a front pocket or zipped pocket. Whilst the vehicles can appear too full, folk do make way for you to enter, just step up and push in. Keep clear of the doors as easy to get trapped.
Enjoy exploring Belgrade - this is the best way.