How do I know if I have cornea ectasia? How common is it after lasik?

Not common. There are special testing which can be done, i would suggest they be done by a corneal specialist. But the hallmark is unstable refractions, caused by unstable or changing corneas.
Ectasia. Ectasia after laser surgery is rare. It causes blurred vision which slowly progresses over time. See your corneal surgeon and corneal thickness and topography can be performed to look for ectasia.