What's Training? is an addon for World of Warcraft Classic that adds a list of available trainer spells to the spell book, so you can easily know when to seek a class trainer out. Spells you already know are moved to the bottom. Mousing over a spell will show you its tooltip, with the cost to train attached to the bottom. Mousing over a category header will show you the total cost of training of spells in that section.
If you have Class Trainer Plus installed, What's Training will show Ignored spells at the bottom.