When should I tell my 13 yr old daughter that I am not her biological father and unsure who the biological father is?

I have been divorced for almost 4 years now.  I have 4 children that live with their mother.  I have known since his birth that my 3rd child (10 years old) is not biologically mine.  When he was 7, I explained to him that I was not his biological father and actually introduced him to his other side of the family even though his mother disagreed with me.  He has learned to live and grow up with knowing the truth and says that I am still his "dad".  The issue I currently deal with is 3 1/2 years ago, I found out that my first born daughter ( from a shotgun wedding) is also not biologically mine.  She is 13 now and I am not 100% sure who the biological father is.  I have an idea but do not want to "assume".  My ex-wife will never have the decency to tell me the truth and I believe my daughter has a right to know.  I tell myself that it doesnt matter- she is my daughter heart and soul, but I think she should know.  My question is: Is she at the right age to be presenting this too and if so, how should I go about accomplishing this task?  I would need to have a DNA test done or blood test and then how do I even know who the biological father is?  I would greatly appreciate anyones insight on this matter.  Thanks!