You could also use a clay bar (if you have one handy) coupled with some detailer spray and microfiber rags. Should take water spots off with ease. I would wipe the surface with cleaner/detailer spray first just to get rid of any loose particles, then hit it with a clay bar and detailing spray, wipe it all down nice and good, then follow that up with a coat of wax. That'll have your glass looking like it's brand new. I really like Meguiar's Ultimate Liquid Wax for glass. If you're glass has swirls or scratches in it you might use some polishing compound before you apply the wax as well to help clear some of that up.

While you're at it do your whole truck! Then the whole thing i'll look new.

 

not sure if someones mentioned it already but try Stoner glass cleaner + micro fibre cloth. If you get spots after a car wash even just use 2 micro fibre cloths.. one to make windows dry and other to get the spots off.. works great.. unless you have dirty windows.. then stick to Stoner glass cleaner, it's safe for tinted windows too