I'm not from A&M but I did check out their website real quick. On the A&M website they have email addresses listed for their officers. I would contact the rush chair for starters and maybe some other members. If all else fails, I am sure the ladies there wear their letters while on campus. So if you happen to see someone, stop them and express your interest. All the SA girls I have met are super nice and I am sure they could tell you when rush is if they have planned it.
I'm glad you are considering Sigma Alpha and good luck!