I'm a pretty friendly person and often I strike up conversation with strangers in queues and the like.

At airports I generally do so with the option of lounge guesting in mind. If the person is friendly and seems pleasant then at some point I bring up lounge access casually (along the lines of "I'm looking forward to getting to the lounge") and see how they respond. Most of the time they say the same, but if they say they don't have access then I offer no-strings entry as my guest. If we don't click then I don't do it.

Everyone has their own moral compass but this fits mine.