How can I know that a theme is responsive for all devices?

[l_martin]

How can I know that a theme is responsive for all devices? (without using any tool)

Why don't you just use the Google's tool?

I am not relying on it because the Google's tool is like yes and no, there is no such thing as moderate or semi in it. Some themes may be not fully mobile optimized but they load and show as good as any other mobile optimized theme.