If the player isn't sure where they sit on the Companion's affinity scale, they can work towards earning affinity and wait for the game to let them know when they've hit a major milestone. The maximum possible affinity is 1000 points, and the lowest possible is -1000 points.
Usually, companions will begin with an affinity level of zero. However, there are a few tools that can be used to keep track too.
Unfortunately, a companion's affinity level is hidden from the player and can only be seen through console commands.