The way I try and do it is to use "Said" when I think the speaker might be unclear, and otherwise I just leave it hanging and figure the reader will work it out based on context. Example...

[quote]
"The bus is late." Joe said, looking down at his watch as if it had just insulted his mother.

"Bus is always late. It's been like that since I moved to this neighborhood." Larry replied, calmly rocking back and forth on his heels.

"I can't be late. Skipped two days of work this week already. If I skip a third..."

"Flu?"

"Yes." Joe said with a sniff. "I shouldn't be going. If I miss today, that's it. I'm done."

"It's been goin' 'round." 

"I know. I don't recommend it."

The two men became quiet for a time. In the fog they looked to each other like outlines, bodies where one man could imagine the other as anything. Another form joined them. It started as a low moan on the street, followed by a high pitched squeal and the appearance by a behemoth of shadow in the fog.

"Ah." Joe said. "It's about time." 
[/quote]

It is really easy in RP writing for a person to assume others aren't actually paying attention. That's probably one of my biggest flaws as a writer; assuming the reader needs everything spelled out for them. But if you worry too much about it, you can end up creating clunky sentences...

...or, god help us all, coloring the text like it were somebody's forgotten myspace page.