So to paraphrase the Supreme Court Justice, "We don't know how to define 'cheating', but we know it when we see it."
Basically indicating is considered "ok" since the tile played is during the game and visible to all players (i.e. the play of the 5:1 tile when the bidder is fishing trumps, you are void, and can play anything tells your partner AND THE OTHER TEAM that you probably have the 5:5"). But anything that clues your partner into the contents of your hand BEFORE bidding is completed is considered bad form by the N42PA. Please do not engage in prompt bidding or similar practices to convey to your partner what suit you are strong in or doubles you may be holding.
There is an online class covering the basic zero to hero 42 rules at classroom.google.com (class code is:mz26l5). Note that the l in the code is little l and not the number 1 or capital I. One other quick note on the class is to use a basic gmail account to access it (corporate or school accounts will use the version of classroom that has silos for those organizations instead of using the public classes which this is).
42 is a social game and is meant to be fun. People get pretty hacked when they pay $40 to play in a winner take all tournament and some teams are prompt bidding. So we know that this is a strategy that can be employed. Please don't do it.