FAS or ADHD?

We are in the process of adopting a 12yo girl. SJ has been diagnosed with ADHD, but we (along with our worker) were questioning the possibility that she has FAS. Although she doesn't have the physical characteristics of FAS, she has the "swiss-cheese" memory that is common with FAS. She had been in foster care for about 2 yrs before she came to us. She has the mentality of about a 5 or 6 yr old child and is maybe a 2nd grade reading and math level at best. We can teach her how to add, subtract and multiply one day, then she claims she has no idea of how to do it a day or two later. Then, she'll surprise us by remembering some small detail from an outing 6 weeks before! She also has the attention span of a much younger child. Even while watching a movie that she picked out and would seem appropriate for her age, she can't pay attention for more than short periods of time, then she disrupts everyone else watching the movie so that she can understand (catch up with) what is going on. Does anyone out there think that our having a test done to help us diagnose FAS would help her with her learning/developmental delays? Also, how could we help her learn/retain information better if we found out she is positive for FAS? Thanks!