I think the simplest way to figure which Stack Exchange site to post on is to perform a Google Search using the title of your question or some keywords from it.

Inevitably, near the top of the results should be some Stack Exchange sites, and probably only one or a few of them.

Visit the help/on-topic page(s) for that/those sites to see if it or one of them sounds like a place where your question is on-topic, and assuming that it does, then post it.

The worst that can happen is that your question gets put On Hold as being off-topic, and in the process you will have started to learn about that site's scope and/or about the type of editing necessary to bring it on-topic there.

I see no need for a site to "test post" questions.

I think the simplest way to figure which Stack Exchange site to post on is to perform a Google Search using the title of your question or some keywords from it.

Inevitably, near the top of the results should be some Stack Exchange sites, and probably only one or a few of them.

Visit the help/on-topic page(s) for that/those sites to see if it or one of them sounds like a place where your question is on-topic, and assuming that it does, then post it.

The worst that can happen is that your question gets Closed as being off-topic, and in the process you will have started to learn about that site's scope and/or about the type of editing necessary to bring it on-topic there.

I see no need for a site to "test post" questions.