Count the material: who is ahead and by how many pawns.
Before every trade or sacrifice a player estimates the material on both sides. Counting quickly helps you see who is winning and not give pieces away.
The trainer shows a position from Stepchess puzzles and you say who is ahead and by how many pawns. After the answer you see the material of each side.
They are roughly equal, 3 pawns each. In a given position one can be stronger, but for counting they are the same.
The king cannot be traded or lost: the game ends without it, so it is not part of the material count.
Yes, the trainer is free and works without sign-up.