Conduct a small social experiment.
Give your baby to a male friend. Not permanently. Just a short term loan.
What does the chap do?
If he starts a gentle but rhythmical bounce (think Beyonce in ‘All the Single Ladies’ but perhaps with less twerking), he’s a dad.
If he freezes, looks slightly terrified and then grins at you as if to say ‘yes, I’m done now’, he’s not. Bless him. Smile beautifically, retrieve the baby and promise you’ll never do it to him again.