Formal:
* Are you comprehending this?
* Is this making sense?
* Do you follow?
* Is this clear?
* Are you grasping the concept?
* Do you have any questions?
Informal:
* Are you getting this?
* Are you with me?
* Do you understand?
* Is this clicking?
* Does this make sense to you?
* Is this all sinking in?
The best alternative depends on the context and your relationship with the person you're speaking to.