How likely is it that a person can have celiac disease and not know it or have no symptoms?

Will probably depend on severity (1) and age (2) of the person.

(1) A teacher I knew had it severely - like if she had a salad with 10 ml of sauce that contained wheat then she'd be violently ill for hours. A lady I worked with though could tolerate traceable amounts like chocolate.

(2) The co-worker wasn't diagnosed until like her 40's.and showed no symptoms until about then

i think it can happen, sometimes ive wondered if i had it cause of a wheat allergy

Very likely.  Some people with celiac disease will get vague digestive symptoms whenever they eat food with gluten.  But with the majority of people with celiac disease, symptoms aren't noticeable until the disease does considerable damage to the intestines.  It can take decades for symptoms to arise in many people.  Its easily detected with a blood test long before symptoms may show up. 

Almost impossible

Yes I am sure but not sure so I don't know.