My editor and I have made our own indicators for speech bubbles, text and illustration notes (since I am also the illustrator we have lots of illo notes so she knows what I have in mind before I do the rough sketches, if you aren't the illustrator, these will be minimal). We put the speech bubbles in italics with a hyphen and character name, narrative text normal, and illustration notes in parentheses. I honestly don't know if there is a standard. I would just make a notation at the top of the ms how you've formatted it.