Generally speaking, all right. You need to make a model of the bulk glass. Apply it to the glass-Ray material the V . It Diffuse slot invest texture drops on glass:
It also invested in Reflect slot, as well as in the Reflection glossiness. One thing to adjust is the percentage of the use of cards in the material. Just podkuriv this value in the scroll Maps.
As well as invest in the Bump slot relief map of drops:
The only thing you should pay attention to here is, as in the previous slot, the percentage use of the cards to the bump (relief) was not excessive😉
Do everything by eye. Just as you like, as long as the glass with drops it will not look the way you want. Here's a look at the old ship's lantern . In the studio does not render on the background grid, where the lamp is on the table. Dirt on the glass make it, in much the same way. These are the same drops on glass, only in your case will be pronounced Bump and transparent glass itself.
There are more sophisticated methods. For example, the creation of the geometry of the droplets. But this is true only if the angles of the subject klozap visualization. For example, when rendering the glass with a cold drink, which is fogged up and it formed droplets😁
If you just need to show the rain outside the window glass in the interior, the version of the material will be the most optimal for you.