I know how to make a GPO to deploy an application. The question is, is there ANY way to check whether that specific thing I want to install already exists on a machine? I assume there isn't but one never knows. I'd like to save on bandwith because of severely limited speed at our customer.

Basically I am trying to come up with a way of updating Flash Player on global level, because users won't do it. Of course, deploying it on machines that already have it sounds stupid.

you could script it, so that you run the script and check the existing version and if older than what you are deploying copy the installer from a network share to the machine and install.

Or you push the current version to everyone, then I believe you can use the upgrade option in the advanced software deployment in GPOs to perform future maintenance.

And in the meantime look into and invest in SCCM.

we've already used SCCM as our deployment platform, easier to monitor and manage at backpoint . You could first script and pacakage source file or app into one new pacakge via tools like "autoit", including script to check app version in regitstry. Then create collection with condition ( here you can use app version or others ) . The app depoyed via SCCM can be reviewed via web view.

SCCM is out of question. Such things will not be used over here anytime soon. I have to stick with what I have available

Then you'll likely have to write it yourself as previously posted, unfortunately. GPO software push is the "barebones" solution, and the recommendation when logic is involved is to move up to SCCM.