I put red dye in our 4 toilets and an hour later each showed the dye in the bowls, to varying intensity.

The toilets are less than a year old.

What can I do before calling a plumber?

Thanks, Jim

Assuming you put the dye in the tank, it means that the flush valve is leaking. There are several types of flush valves but most toilets have the flapper type. How to change a flapper should be obvious to you. Just make sure you leave enough slack in the chain for the flapper to close completely. Unusual for four toilets to leak in one year but perhaps the manufacturer didn't use the best grade of flapper. Any nationally known brand of flapper is good.

Agree but tell us what was happening that caused you to start looking at the toilets