ETTL Flash question

Dear DPexperts...

Is it possible to know at what power the flash fired under ETTL mode ? I would like to practice with off camera flash, but at this point I don't want to invest on an expensive ETTL wireless flash. I'm not planning / not sure if this would be useful for me and in what situation I would really need that. I'm planning to invest next on a 600EX-RT (I just got my 5D MKiii and having a blast with pictures at high ISOs - indoor sports mainly). I currently own a 430EX-II.

I can calculate exposure - read several articles online about that - but just trying to see if there are any other options as well.

Thanks again for any feedback.

Cheers