The faint sign of tracks on the ground... and that the tram is 50% longer than the bus. That's not minor. It's crazy that they went to the extent of designing the bus and the tram to look identical, then just lobbed on another 50% length to the rail.
There are also experiential differences between bus and tram, at least here in my city (which has the most heavily-trammed road in the world). A tram pulls up, and punters can get on from a variety of doors. A bus pulls up, and you can only get on at the front, and have to shuffle your way down the vehicle. Buses are usually a lot more cramped as well - few buses handle a stroller well, though some of the newer ones do.
In any case, if you're running your buses like trams, with their own dedicated lane and so forth... then you may as well turn them into rail vehicles, because they're cheaper to run (both in maintenance and wages) and service more people. Of the two images presented, each vehicle has one driver, but the tram carries 50% more folks.