Tomato intolerance is a type of food intolerance specific to Tomatoes.  It is usually because the body doesn’t produce enough of a particular chemical or enzyme needed for digestion of the salicylates present in tomatoes. Even though eating tomatoes will not cause an immediate life-threatening allergic reaction in people who suffer with tomato intolerance, the lack of a specific enzyme in the body may also lead to the build up of toxic byproducts and histamine which can mimic the symptoms of an allergy.
So what are the symptoms?

Tomato intolerance is rarely harmful but can cause very unpleasant symptoms to the sufferer which can begin soon after, or several days after eating tomatoes.  Symptoms can include: nausea, bloating, abdominal pain, intestinal contractions, diarrhoea, flushing of the skin, palpitations, headache, urticaria (hives), and in extreme cases where there is a build up of histamine resulting in a pseudo allergic reaction, even difficulty breathing (in which case, you should seek immediate medical attention).  If you often suffer with any of these symptoms, I would strongly recommend you keep a strict food diary to help identify the trigger food.

How can it be diagnosed?

The only really accurate way to test for tomato intolerance is to remove tomatoes from your diet (including any products that may contain tomatoes as an ingredient – which can be quite tricky to do), see if the symptoms improve and then reintroduce tomatoes.  If the symptoms return, an intolerance is very likely.