I'd like to know if I claim an adult dependent (meeting all criteria) if that dependent owes back taxes -- would it make me liable? And when claiming do I have to show what back taxes they owe?
For your information, I have never known the IRS denying a taxpayer a dependent exemption for an individual who owes back taxes, nor have I ever seen the taxpayer being personally held liable for the back taxes for the dependent claimed on a taxpayers tax return!
My recommendation is if you qualify to claim your adult dependent based on the dependency rules per the IRS tax code, then go ahead and claim the person as a dependent.