How to know through my website if my software is installed on the client

One of the requirements of my website is to run explorer in order to open a local folder on the client. This obviously isn't really legitimate so I solved it by defining a custom protocol. This means that my website contains something like this:

document.location.href = "customproto:" + local_path (The reason I don't just use href in my html is irrelevant to the issue)

In order for this to work, every client installs my software that does two things: 1. Put an exe file in a certain location i.e C:\temp 2. Add some keys to the registry that connect the custom protocol to the exe.

In my website, I need to know if the client has the software installed, so I can tell the clients who don't have it to install it. I thought about a few solutions, but I'm looking for a better way. 1. I could add a field in my DB and update it when a user installs the software. 2. I could use the local storage of the browser. 3. I could try and find a loophole or some hack.

I'll be more than happy to hear your ideas.