When an RP fic comes from a transcript, it often uses *asterisks* or other methods to indicate action, rather than writing out a complete narrative. It is easy to spot RP fics when the person who uploaded them did not bother to change to a narrative style and left it in the original script format.
Roleplays that were completely written out originally appear more similar to collaborative efforts and are not as apparent as script-style RPs when they are used as a fanfic. However, it depends entirely on the skill level of the players and their ability to create paragraphs.
Due to the general specialness of most RP characters, nearly all RP fics have Sues in them of one sort or another.