In your case you could put a sort of tag at the end of your question:'Do you have ... ? Be honest/Tell the truth/Don't pull your punches' (the last is informal). Or you could use a similar command (or condition) before your question: 'Be honest - Do you ...? or 'Being honest, will you tell me....? Or, jokingly you could add 'I'm a big boy now - I can take it.'