I'm sorry I missed it. Do you mean asking someone's church?
As far as I can tell, the ONLY reasons a person would have for not being willing to identify a Protestant denomination is that they don’t want to be pinned down to any theology OR that they aren’t actually Christians in the first place (and there are several on this thread who I am fairly certain fall into this category).
There are denominations with only one or two churches in an area. Know their denomination and where they live, and you’re on your way to being able to compromise their screen name.
Besides, the mods have slapped people down in the recent past for that and called it trolling.