I'd like a way of knowing quickly which emails i've replied to. Is there an icon or something?

Something like what Apple Mail does (see attached).

Having the conversation number is useful but if the conversation gets large I have no way of telling whether i've replied to it unless I go into the email and have a look. This can be really time consuming. 

Also, I know I can flag emails so I could do it manually, but again with the number of emails I have to deal with it would be too time consuming.

Thanks for your time