Can I find out who claimed my son as a dependant on their taxes.

I gather you filed electronically. First check and make sure you entered his SSN correctly. If so, what I think you need to do is file a paper return with a cover letter stating that you are the only one entitled to claim him. If you file electronically, no person looks at it, so since the SSN was already claimed, it was automatically rejected.

The IRS won't tell you who, but they will resolve the issue.

Are you sure it wasn't the mother who claimed him?

According to the IRS, if multiple people claim the same person as a dependent they determine who the rightful person is based on the following:

1. The person who is the parent.
2. If both parents claim, then the parent with whom the child lived with longer.
3. If the child lived with both parents an equal amount of time, then the parent with the higher AGI.

This assumes that you and your ex don't have a divorce decree that gives her the right to claim your child.

Hee is a link to IRS Pub 501 that covers this - see Table 6 on page 11:
http://www.irs.gov/pub/irs-pdf/p501.pdf

From this it would appear that either you or your ex are entitled to claim him, but since you made more money than she your claim outweighs hers. Your claim also outweighs anyone else's. I suggest that you file a paper tax return, and include an explanation stating that the dependent is your child and he lived with you throughout 2007. Make sure you use the correct SSN for your son.