How about leash walk her to go potty. Its going to be an effort on your part, but at least you'd know.
I know if my dog was sick like that, instead of complaining about not being able to find out if she's sick, id walk her. You'll know for sure if she's indeed blocked, or if its something else.
Blockage is painful for dogs. If she's not passing anything, then she's probably in pain. Don't you think it'd be in favor to your dog to hand walk her?