With the sudden onset and not knowing of any exposure to new items the general rules are as follows since both show similar rash patterns, itching, etc.
Individuals with allergic contact dermatitis typically develop dermatitis, within a few days of exposure, in areas that were exposed directly to the allergen. Certain allergens (eg, neomycin) penetrate intact skin poorly, and the onset of dermatitis may be delayed up to a week following exposure.
A minimum of 10 days is required for individuals to develop specific sensitivity to a new contactant. For example, an individual who never has been sensitized to poison ivy may develop only a mild dermatitis 2 weeks following the initial exposure but typically develops severe dermatitis within 1-2 days of the second and subsequent exposures. The immediate onset of dermatitis following initial exposure to material suggests either a cross-sensitization reaction, prior forgotten exposure to the substance, or nonspecific irritant contact dermatitis provoked by the agent in question.
Immediate reactions, ie, visible lesions developing less than 30 minutes after exposure, indicate contact urticaria "hives" (not allergic contact dermatitis). This is particularly true if the lesions are urticarial in appearance and if the skin reaction is associated with other symptoms, such as distant urticaria, wheezing, ophthalmedema, rhinorrhea, or anaphylaxis.
So basically the easiest way to know with some certainty is that if it is a sudden onset without exposure to a new or know substance you are allergic too it is likely hives and if it does cause other symptoms in the body this reinforces it being hives.
If there has been no new or know exposures then it is likely a delayed reaction and in most cases contact dermatitis.