Use this tool to check whether your itinerary can be e-ticketed in the "normal" way, i.e. according to the ATPCO validating carrier selection logic.
Simply type in your itinerary as given to you by the OTA, leg by leg: the 2-letter airline code (e.g. "AA" for American Airlines), and the 3-letter origin and destination airport codes (e.g. "JFK" or "LHR") for each flight segment. You'll find these codes on your itinerary or reservation. Note that we don't need to know the dates or flight numbers or any other details.
The tool only performs the logic a travel agent's system would to determine the validating carrier. It doesn't check whether the airline you enter really flies the route you supply or perform any other kind of checks. The results are merely of an informative nature; you should always do your own due diligence before buying an airline ticket.