If you have been the custodial parent since June 2010 then you are entitled to claim your children on yuor 2011 taxes, eithout question. The foster parents may have been able to claim them for any year that the children lived with them at least 6 months. But from what you're telling us the children spent no time with them in 2011, and hence the foster parents may not claim them as dependents.
If someone else is claiming your children there is no way for you to find out who it is from the IRS, because all tax returns are confidential. If you are finding that your return can't be e-filed because youor dependents have already been claimed by someone else, go ahead and file a paper return instead. Claim the children as your dependents. and attach a cover letter to explain that you were the custodial parent for 2011. The IRS wil process yor return as normal, and later on will investigate.