What are the visual differences between Trent 892 and 900 engines?
The main difference between the two installed engines would be the fan blades. The 892 uses hollow wide chord Ti blades, like shown below.



By Cory W. Watts from Madison, Wisconsin, United States of America - Rolls Royce Trent 892 [777-200ER], CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=43524027

The Trent 900, on the other hand, uses a swept design to reduces the effect of shock waves. From RR site:


  The engine’s 24 fan blades are to a new swept design that reduces the effect of shock waves, as the tip of the fan rotates supersonically, making it lighter, quieter and more efficient




Image from imagesourcemedia.com; appears to be taken from RR site.

Also, the Trent 900 is exclusively for A380.