i take real world projects with me and do them in the store.
so make a 4 patch and check for straight seam that are 1/4 each. test to see have it sews over the seams when putting the block together.
bring in a quilt sandwich and test the walking foot and the free motion quilting.
have an applique block to test the various machine applique stitches.
i make handbags often so i bring an almost finished bag and i finish it to test its ability to sew over thick seams.
so i'm sure you get the idea - bring in samples of the type of sewing you do and plan on doing in the future and test the machine to its fullest.