Can 900MHz and 2.45GHz Antennas be differentiated by shape?
It's not entirely clear what you're asking...

The only real question in there I can find is:


  Can the two types (usually) be distinguished by appearance?


To which, the answer is no. The overmolding is not indicative of the shape of the wire inside and is decorated for design/branding reasons. If this were not the case, it would look like a wire (obviously).

Here's the classic "rubber duck" antenna. It's a normal-mode helix, in this case targeting ~850MHz (US cellular band)



Some other questions are implied like:


  Does the shape/length tell you the target frequency band?


Also, generally no, but the there are very real limits to size -> frequency band with any practical efficiency so, with some caveats, you can make some (very) educated guesses as what was intended. The biggest giveaway is the dielectric medium. In free-air, the practical lengths are in a certain length-to-band range, in ceramic media, the practical lengths are different but also in a limited range.


  I'm pretty sure there are some passive elements in there as well. To increase the useable bandwidth perhaps.


To which the community responded that this is not the case. The reason is that the passive element to which you were referring (based on your later edits) is a loading coil, not a matching element. It doesn't increase the usable bandwidth (in most cases it actually decreases it). Instead, it moves the center frequency lower and lowers the radiating efficiency in the process.

Loading coils for short antennas feature high reactance values and extremely low capacitance at one end relative to the other. This makes the coil vulnerable to environmental capacitances as they will be differential to each turn of the coil increasing circulating currents. 

Turn-to-turn capacitance increases effective inductance, effective resistance, yet reduces system bandwidth and ironically quality factor!

Charles Rauch summarizes this nicely:


  In tank circuits, or with loading coils used with longer antennas or
  with capacitance hats, the optimum inductor shape becomes shorter and
  larger in diameter. 
  
  In a short mobile antenna or very high impedance tank system or
  circuit, optimum inductor form factor becomes longer when compared to
  diameter. Long high inductance coils are generally good, compact large
  diameter inductors often work better in low inductance ranges.


So most of the engineers didn't consider your claim of "passive components" in the overmolding to include loading coils as "inductors" since long helice behave more like a radiating transmission line than a conventional inductor.