Bubbles are not normal ... they indicate weak or blown gasket ...
For further clarification search signs of a bad gasket and you will find tons of videos and every video would have bubbles making as one of the indications of bad or blown gasket ... my mechanic also used to check my car radiator for bubbles to indicate that ...
The normal way is, remove the cap of radiator and press accelerator if with acceleration bubbles increase or atleast are forming then it is gasket ...
Under normal conditions a healthy engine never produces bubbles in radiator ...