Or, when you go to sell them back, display a list of all spells 'bought' with training points, so we know exactly what we are selling back.
Hit 'P' to open your spellbook.
Any spell there not from your school was purchased with a training point.
You could take a screen shot by hitting PrtScrn and pasting it into Paint, or you could use an ancient Chinese technique for getting the information from your computer screen:
