Skill Checks are some challenges you can do in the game in order to unlock more characters, cosmetics, stages, etc. For this achievement, you need to get 50, a lot of these you can easily get right at the start of the game, it's possible to get over 20 skill checks right at the first race. Just pause the game and check for any skill checks you can do.
Here's a Skill Check Speedrun for the game so you can have a decent idea on which Skill Checks you can get easily:
As well as a guide for a few Air Tricks Skill Checks:
And lastly, a video showing all Skill Checks:
Once you get the hang of it, it's not too bad, you can easily get 50 Skill Checks right at the start of the game, good luck!