“How do I know if my child has got an allergy or conjunctivitis?”

It depends on the cause. Visit your ophthalmologist for specific diagnosis.

It could be viral conjunctivitis which is quite contagious and spread when one kid gets it and spreads it to their friends It could also possibly be allergy if contacted with something he was allergic too. In a child I would see a pediatric ophthalmologist to get checked

Based on the symptoms, there is no way to tell. He would need to be examined. Don't play around and assume it is allergies, because if you are wrong, he can spread it to the whole family and his friends.

If both eyes got involved at the same time, and there is no purulent discharge, most probably its allergy. It should respond to cold compress and allergy medicine. Otherwise you better to see an eye doctor for further evaluation.

In the early stages, it is often a challenge to tell them apart.

Some clues that this is probably allergic conjunctivitis: history of other allergy signs (itchy nose, sneezing, asthma-like symptoms, constant rubbing of eyes, pets in the house, seasonal occurrence

Clues that this is more likely infectious conjunctivitis: acute (sudden) onset, usually starts in one eye, mucus discharge (pus-like), fairly swollen lids (which may swell shut), extremely bloodshot (might even have splotches due to broken blood vessels, pain, light-sensitivity, cases of pink-eye going around the school or community.

If it is allergic, using topical allergy eyedrops and/or antihistamine like Benadryl, cold compresses, it will usually be better the next day.

If it is infectious conjunctivitis, it will get worse before it gets better, no matter what you treat it with. Note that, although primary care doctors will usually prescribe them, antibiotic drops will not cure most cases of conjunctivitis since they are due to viruses (which are not affected by antibiotics) and not bacteria.

In infants or children under 2, it is not inappropriate to use antibiotics as protection against bacteria which can coexist in infections since children this age don't have the same immunity.

In either case you can use a decongestant/antihistamine drop such as Naphcon-A up to 4 times a day. Again, if it is allergic it will get better pretty quickly. If it is an infection, it will get worse before it gets better and can last up to a week. In severe cases, the eye doctor might prescribe steroid drops but this should be used with extreme caution as it can cause glaucoma and other possible complications.

Children should be kept from school if they have an infection or if the parents aren't sure.

There are multiple possible diagnoses: juvenile iritis, allergic conjunctivitis, viral conjunctivitis, bacterial conjunctivitis, foreign body or abrasion to his eyes, to name a few. Please take your child to a competent ophthalmologist (MD; not OD).

Allergies will be recurrent, worse with rubbing and associated with asthma and eczema. Time will tell.
Conjunctivitis is usually one off. Keeping good hygiene will avoid recurrence.

Most commonly, an allergy symptom is itching. However, the symptoms noted can be common with both allergy and conjunctivitis. An examination by an eye care provider can determine the cause of your symptoms.

Allergic conjunctivitis is a common form of conjunctivitis; there are many forms. If it causes persistent symptoms, an exam can determine the cause of his conjunctivitis, which will then dictate appropriate treatment.