Here is your first real opportunity for assessment. Show a group of children how to play this game. Once they are able to play independently, you can observe and see how much information students are retaining. This is also an opportunity for the children to practice and internalize the skills that you are learning.
For this game, you will need a couple of magnets, the materials you collected for the last activity, dice, index cards, something to make a game board out of and game pieces.
Draw or trace pictures of your materials on index cards. Then make a game board in which every other space has a picture of a magnet. In the remaining spaces, you can either write a direction for the children to follow, such as go back one space, lose your turn, etc., or you could just leave those spaces blank. Once the game board and cards are prepared, you are ready to begin. Have the children roll the dice and move their game piece the correct number of spaces. If the child lands on a magnet space, they can draw a card and try to predict whether the object on the card is magnetic or not. After they have made their prediction, they are to take a magnet and try to pick up the object depicted on the card to see if their prediction was correct. If the child was correct, they get to hold on to the card they chose, and will roll the dice again on their next turn. If the child was incorrect, they are to put their card at the bottom of the card pile (hopefully to be reused later in the game). On their next turn they will not roll the dice, they just choose another card. First to the finish line "wins." If you would prefer to not have a competitive game, you could make the board a circle, so children can continue going around it as long as they are interested. (Day, 18)
This activity can easily be reintroduced periodically through out the unit as a type of review or assessment.