How to achieve dusty looking on a surface, making a particle system was my first opinion but then I changed my mind!
It would've cost too much and now I'm stuck.
There are a couple ways of doing this. Here's one way in cycles as your question didn't specify which engine you are using.
Start by adding an input>geometry node using the normal output.
Next, add a Separate XYZ node, you will be using the Z output.
A color ramp will be next, this will control the mix between the dust layer and the rest of the material. Plug this and the preceding 2 nodes into the Fac of the mix shader as shown below.
This is a suzanne with the above material: