How can I tell how replayable a cooperative game is?
It's tricky to evaluate a game's replayability before buying it, as the primary question I ask to assess this is: 

"Does the game support multiple viable paths to victory?"

My thinking is this: If a game has one optimal path to victory, repeated plays will likely become boring (since you're just doing the same thing over and over again).  To figure this out, I typically need to play a game at least once, carefully assessing the opportunities and tools that the game presents during play.

If I couldn't play the game, I'd try to read the rules and look over any primary game components (like card decks, character roles, etc.).  I'd also check out reviews, of course.

The second question I ask is:

"Does this game use one or more modular/randomizing components as a core component?"

Do you lay out tiles to form a different board each game?  Is the game driven by one or more decks of cards with critical goals or events shuffled in at semi-random spots?  Is there a character creation system, or are the characters the same from game to game? -- All of these things contribute to replayability.

Again, it's nice to play the game to see all the components interact with each other.  However, this is a bit easier to assess from just looking at the box or reading the list of parts.  It's also easy to assess from reading the "set up" portion of the rules.