"How can I tell if [he] used us on taxes this year "
You can't directly know if you and your son and baby were used on another return, and the IRS won't tell you (because of privacy reasons).
However, you can determine this indirectly by filing a return and stating that you were not a dependent as well as claiming your son and your baby as dependents on your return.
If someone also claimed you and your kids as dependents, then your return will be rejected with a reject code that will point to your and their Social Security numbers already having been used.
In this case, you should "paperfile" your return (print and mail it), and eventually the IRS will notice this and write letters to you and to the other person claiming you and/or your kids, asking for proof of the right to claim these dependents. Then the IRS will use one return and adjust the other return as necessary to remove the kids.
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