How do I tell if someone has been poisoned ?

How can I tell if one of my patients has been affected by CO?

Well, it can be difficult. As always, an accurate history is the best starting place. That, and thinking of the possibility in the first place.
For some reason, doctors when faced with more than one person exhibiting similar symptoms thinks 'infection'. Try thinking 'toxic gas' instead, or as well!
Are you looking at more than one member of the household or building occupants with the same or similar symptoms?
Are any PETS in the house behaving oddly too? (Smaller animals, as well as children, are more susceptible than fully-grown adults.)
Do they have any way they could be being exposed to CO? This includes the obvious like gas fires and gas-powered boilers, but also ANY fuel-burning appliance – wood, coal, charcoal, paraffin, oil, etc.
Do they have an adjoining wall or shared chimney with a household that does have any of the above – even if their house does not?
Have they had their gas appliances checked regularly? Have they had their chimneys swept?

One way of 'proving' exposure is to check their COHb level, but you must bear in mind the length of time elapsed between exposure and testing. Read this post for more on this.