What age should we check allergies for my toddler?

No set of age: First of all, allergy testing is not indicated for an asymptomatic child. If your child has had suggested symptoms of allergy such as eczema, food intolerance, or recurrent wheezing, allergy testing would be considered, but not mandatory.

Some basics: Much of what your interest is will determine the approach to evaluation. Food allergies can be assessed to some degree with blood tests & foods are something you can control exposure to. Airborne/outside allergies can be managed some with meds & you cannot control/eliminate exposures. Insurers rarely cover skin tests for these until 4yo. Discuss it with your doc & go from there.

More details would be helpful. If your family members have a history of allergic disease, the risk for development of allergies and possibly asthma are higher for your child. Very often the first manifestations include hives, eczema followed by a runny nose when not infected, and then cough/wheezing. There are many lab based tests which may support the diagnosis and guide avoidance.