How do I know when the battery is full?

That looks like a vivosport. I don't have one so I can't say for sure, but I got a vivosmart 3 which is pretty similar looking. You should see a charging symbol while charging. You should also be able to change the home screen to one with a battery indicator.

Hi !  somewhere I see it´s charging...(I really dont remember) , but then I have to guess if the battery is full. the home screen is the front part of the watch ? or is it on my cell phone?  Thanks for taking the time!!!

The right word is rather watch face. Yes. The screen that shows the time. 

I've moved this thread to the vivosport forum, since this is not a Connect for iOS issue.

Thanks for your “time”