By your first ultrasound. Also genetically speaking identical twins are a fluke where the cell splits and fraternal twins are only passed through the MOTHERS side. Meaning that even if fraternal twins are common on your husbands side it wouldn't effect you.
they would be able to tell from your hcg levels as well as listening to the heart beat with the doppler and/or an u/s.
Wat is a doppler? Do they insert it?