Getting Twins tested for ID/Non ID

Anyone recommend anyone/anywhere who can test twins to see if they are id or non id they were never 100% sure with my boys think id, just wondering who to ask to get information hubby heard that if the test is less than ??100 it not really reliable. Any suggestions. Just want to knwo out of curiosity and people always asking.

Max and Milo 28 weeks and Luke nearly 20 months