Checkers board game stages
There are very few specific board setups that detail early- mid- and late-games. It might be easiest to keep track of which stage you are in based on how many pieces and how many kings each player has and comparing to one another.

For instance, when very few pieces remain on either side, regardless of ratio of kings, this can be assumed to be the late game. Another example of late game is when one player is being dominated and has about one-third to one-half of the pieces that the other player has.

In the early-game, as another answer mentioned, some combinations are easy to memorize, especially if you play a lot. In the late game, some strong finishes are to be known in general, such as having all enemy pieces in a position where none of them can move (e.g. have your pieces block off your rear end) or sweeping the board with kings.

Mid-stage is more or less defined as the messy stage, where you're making up every move as you go and taking time to decide based on how you think the other player will react and how far ahead you can plan, aiming for the late stage. The two sides are approximately even and players are vying for the most profitable positions.