Who else did the child live with in 2016 and for how long? How long did the child live with you and how long did the child live with your girlfriend? Did you live together and how long?
Residency and relationship are more important than support when it comes to claiming children.
There is no way to check to see if someone else claimed your child. You just file claiming him/her. If someone claimed him first, your e-file will be rejected. You still won't now who claimed him. You would then have to file a paper return filing a competing claim. The IRS will straighten it out.
Here's the rules:
A child closely related to a taxpayer can be a “Qualifying Child (QC)” dependent, regardless of the child's income, if:
1. He is under age 19, or under 24 if a full time student for at least 5 months of the year, or is totally & permanently disabled
2. He did not provide more than 1/2 his own support
3. He lived with the relative (including temporary absences) for more than half the year
4. He is younger than the relative (not applicable for a disabled child)
5. If the child meets the rules to be a qualifying child of more than one person, you must be the person entitled to claim the child as a qualifying child (this essentially means that you have the parent’s permission to claim the child, if the child also lived with the parent more than half the year)
6. If the parents of a child can claim the child as a qualifying child but no parent so claims the child, no one else can claim the child as a qualifying child unless that person's adjusted gross income (AGI) is higher than the highest AGI of any of the child's parents who can claim the child.
There is a special rule in the case of divorced & separated (including never married) parents. When the non-custodial parent is claiming the child as a dependent/exemption/child tax credit; the custodial parent is still allowed to claim the same child for Earned Income Credit, Head of Household filing status, and day care credit. This "splitting of the child" is not available to parents who lived together at any time during the last 6 months of the year; then only one of you can claim the child for any tax reasons. The tax benefits may not be split in any other manner.
Note in particular that the non-custodial parent can never claim the Earned Income Credit, Head of Household filing status or the day care credit, based on that child , even when the custodial parent has released the exemption to him.