Every jurisdiction has different laws, so the best course is to check with local authorities first. Having said that, I'll talk about my area. A Flare Pistol can be considered a firearm in Missouri, so all the firearm laws apply.
If I came across one in the course of my duties, I would look at the total situtation. Does the person appear to be what he says he is, i.e. someone enjoying the outdoors with other outdoor gear or does he appear to be a thug. If he only had the pistol and flares I would lean towards it being used for its manufactured purpose. My thoughts would go along the lines of: Does it appear to be a signaling device(blaze orange color, made of plastic, etc.)or does it appear to be a "sawed off" shotgun? Does the person also have normal shotshells that could be used in the pistol? If he does and has no other shotgun I would view the item as either a "sawed off" shotgun or a smooth bore pistol, both illegal weapons.
I belive in the individual's right to lawfully have and use firearms(NRA member), so I try to give the benifit of the doubt to the person.
Hope this helps.