Put a scale on your counter and weigh them? LOL, I don't know... But to be honest, I have done that before.
Anyways... Don't underestimate the weight in your boobs. I've always had big boobs, then gained a lot of weight and then lost a lot of weight. I was sure my boobs would stay the way they were. Well...... Not so much and it wasn't until the end of my weight loss that my boobs actually went bye bye. I'm not small now but I'm definitely not what I once was.
If I were to guess though, I'd say they weighed about 7-8 pounds each. LOL