There may be a correlation between a wheat allergy and a barley allergy. If you are allergic to wheat, be sure to test for a barley allergy also. There is no need to assume you are allergic to barley in this case as it is not universal.
You may know you have a barley allergy because you have had an allergy test, or because you have used an elimination diet to determine this.
If you have not used one of the the methods above, why do you think you might have a barely allergy?
If it is because you are tired after drinking beer (or because you have other symptoms caused by drinking beer, besides the obvious ones caused by alcohol) there is another, more likely causes of your symptoms.
For information about avoiding barely, see Barely Allergy – Avoiding Barely.