I introduce my character having a flashback, he's moping around the house remembering how his wife died - there's maybe 10 lines before the reader realises the flashback. Then he hoovers the carpet, puts his clothes away etc and thinks back again for another 10 lines. I am wondering if this is too long. how do I tell the reader he is thinking back. I mean, if it was a comic strip he'd have thought bubbles - do I use italics, bold, underline? It bugs me...
No Italic or Bold, just show the reader it is a flashback and if you can pull it off, make the reader want to follow this flashback and be part of it. Usually when you start a book telling the reader, it will be boring to read. So, to avoid that, try to make the reader live it, make the reader itch. You said the character is cleaning the house, make him realize that he's cleaning a completely clean surface only to curse himself for falling into the trap of remembering. Or, describe how difficult it is for him to clean these things and not remember how his wife used to do it. What I am trying to say is, show the reader that your character is remembering, not tell them in phrases like: he recalled, he remembered, he couldn't help thinking...etc. Hope this helped.
