That's a touch one, Vance - especially once it's gotten dirty.
If the glass is going to be in an area where it may be touched or get dirty, I give them the option of paying additional for me to treat the surface before it goes out my door. The product I've used several times is called ClearShield (http://www.clearshieldonline.com/
) The name Clear Shield is used for a number of different things, even another company which promotes their product for protecting glass, but this is the only one I've had experience with and I've been happy with the results.
In my experience, the treatment alters the appearance almost imperceptibly, and the protection it gives is worthwhile. There are a few steps involved but there are ample instructions and I've found the phone support to be friendly and very helpful.
If you go to the website, you'll see a lot of additional information related to sandblasted glass, after-care, cleaning, etc.
Of course, my information is proactive and preventative, whereas it sounds like you are looking for something in the field which needs to be cleaned. Hope this helps for future jobs.