Are both the sire and dam of the calves registered with American Angus? If not, this is probably going to get more complicated than it is worth. If both are registered animals, and the bulls in question were purchased from a reputable seedstock breeder, then there is a good chance they already have DNA on file with AAA. Get their registration numbers and look them up. I can help you do that, if you have trouble. If that is the case, it will not be difficult to identify the sire once you have a blood or tissue sample from your heifers. You will still need their breeder to transfer them into your name.
If the bulls are registered and don't have DNA on file you will need to get a blood or tissue sample from any bulls in question, and pay for the DNA analysis on them and on your heifers. Parentage tests are $18 and registration is $12 for members for calves under 4 months and that price increases as they get older, topping out at $32 for a calf over 12 months.. Membership is $80 per year. The breeder of the calves will also need to complete a transfer to you and there will be a fee for that. There is quite a bit of information on the home page of American Angus under member services. I believe you can register without being a member at a higher fee, but I don't know what that is.
Normally the animals breeder will do all of this for you and will pay all fees. I'm assuming you purchased them as commercial calves and the breeder made no guarantees of registration. I have a hunch you could buy a registered heifer calf from me with all data out of parents with DNA enhanced EPDs for less or about the same money. I kind of wonder if all the effort and money it will take to get them registered is worth it for calves from a breeder who probably isn't even turning in data to the registry.