Your question is best directed to the Immigration forum. Although I do not handle many Immigration law matters. However, generally when an applicant's parents have died, I've had the applicant list the parent's date and place of birth followed by "Deceased, 1/1/2001" in the current residence box.
Well, you could always ask them? However, the only way to be sure is to contact the IRS in late 2010. Simply file your own tax return (1040) for 2009, claiming your own exemption. Assuming your mother and step-father have not provided the majority of your support during 2009, you are entitled to your own exemption. If your parents claim you and later face an exam (audit), they won't be able to substantiate claiming your exemption.
1 lawyer agreed with this answer