1. find out what company did the freezemark (farmkey or freezemark - they have slightly different styles - I'm sure someone can explain this but I can't lol)
2. get in touch with the freezemark company via their website. they can tell you where and when the horse was registered and any other info they may have - registered name, breed, age etc.
3. for a fee, they can contact the last registered owners on your behalf. Whether they choose to get in touch with you is up to them, but you can but try
If it is not your horse, very little. You can contact the company that did the Freezemark who will pas on your details to previous owner/s, but whether or not the previous owners respond or not is down to luck.
You can contact either Freezemark or Farmkey using their contact information on their web page, depending on the mark although the Horsewatch organisation Horse Alliance owns the database for this the companies keep them up to date.
They can advise if the horse you are looking at is stolen or not. This is the only instant method to prove ownership - or non ownership if the horse is stolen as it can be seen instantly and is preferred by many Police forces. Not all police forces or auctions have microchipping detectors that can detect all microchips which is the other method of permanently securing proof of ownership.
All intelligent horseowners will have their horse / pony freezemarked to reduce the possibility of theft and tracebility if the worst happens. All foals born after 1st July 2009 and all non passported animals will require a microchip before a passport can be issued. The safest option is to have both ensuring the information is updated when ownership changes.
Your local Horsewatch group can arrange discounted freezmarking via Freezemark. See Horsewatch Northants website or any other Horsewatch site to obtain futher details. Horsewatch Northants can scan horses in Northants and surrounding areas with their detector for a small donation to funds.