Unfortunately not many know about this. In my case, I was diagnosed by being able to show images and describe the conditions that were causing the reactions and my allergist was happy to diagnose me based of this information. Some are tested by having a cold compress placed on their arm for a period of time to instigate a reaction.
If you have cold urticaria, then you definately start joining the dots fairly quickly. Over time things are no longer a coincidence.
Allergists and GP's are really important here and if they haven't heard of the condition, go in armed with information, photos and a diary if possible to assist your reasoning.