Allergies and colds work in very similar ways to impede equalizing sinuses and ears. Thick mucus and swollen tissues narrow or block the Eustachian tube and the openings into the sinuses. Severe congestion from either cause can make diving impossible.
However, unlike colds, allergies are amenable to a couple of treatments that work well and have minimal implications for diving. First are the non-drowsy antihistamines (like Claritin, Zyrtec and Allegra). Although these are systemic medications, they typically don't have much effect on alertness or thinking (although one should always try a medication on land before considering its use while diving). Second are the nasal steroid sprays. These work topically, and do not have the rebound effect that nasal decongestants have. They may require several days of treatment to get maximal effect, however, and they are prescription drugs.
As far as assessing whether your congestion is too severe to dive, you can try equalizing your ears on land. If this is easy, then at least your ears are likely to be OK. If you can't pressurize your ears on land, you are very unlikely to be successful at it underwater.
