Burns can be tricky and it's sometimes hard to tell if it's minor or if a veterinarian should be consulted. For starters, if you are unsure then error on the side of caution. Signs that it is serious are:
If any other symptoms occur, like anorexia
If the skin is broken, or breaks in a day or two
If a discharge occurs, clear or otherwise
If a discoloration further than a slight pinkening of the skin occurs
If the "burn" seems to be spreading or doesn't show signs of improvement.
Things to consider if any of the above have/do occur; The skin and immune system are compromised, thus infection will set in and will likely quickly turn systemic. Also burns loose the ability to control osmotic balance, thus fluids are lost through discharge or evaporation and thus dehydration accelerates, even if access to water is readily available. Small burns to us, may be huge on a small reptile.
In your case, you mentioned that there is a burn on the tail. It is quite likely that the worst of the burn is on the tail, and that the rest of the belly also has some compromised tissue.
Good luck with your snake