You can see a member's reputation easily by the number of "gems" that are displayed below their avatar, on each of their posts. The Green gems indicate a positive reputation, the gray gem indicates an unknown reputation, and red gems indicate a bad/negative reputation.
How does the number of gems reflect the member's rep point count?
Each rep gem represents one "level" of reputation. The list of levels is below:
If you hover the mouse over the rep gems you will see a tooltip showing their number of points (to the nearest group).
It is also possible to see someone with a black gem. This indicates that their reputation score has been disabled. Generally this applies only to admins.