When partner does not lead your suit, assume partner is void, or has a side suit singleton, or has a side suit sequence, like an outside AK to look at dummy, or has forgotten the bidding!
When partner overcalls it is usually a "Lead-Directing" suit, not necessarily to buy the contract, but to get partner to lead that suit.
When partner does not double an artificial bid (Stayman, Transfers, Cue-bids) particularly at a high level use the inferences of what to lead.