Is assuming someone isn't Muslim takfir?

[ehersomeh]

When you falsely assume that someone isn't Muslim, but then find out that he is, is this takfiring?

[Eisenman]

For me, it is not. That can be a simple mistake that anyone could make.

[Eric H]

When you falsely assume that someone isn't Muslim,

We are probably better off when we judge ourselves rather than someone else. We are in a better position to change ourselves; than we are to change others.

[Ümit]

When you falsely assume that someone isn't Muslim, but then find out that he is, is this takfiring?

İ dont know what you mean with takfiring...but it is wrong.
You cannot assume someones religion based on their looks.

So please dont do that.