During the progress review, how might a project manager assess the contribution of each member of the project team?

During a progress review, that is what the job of a project manager is to do--review the progress of each member on the current project. Some ways they could do this is first off by asking what milestones/deliverable did they have a stake in working on. By doing this, the PM will be able to find out all of the milestones they helped accomplish and what other project members had a hand in completing, as well. That way, when reviewing the progress of other team members, they will be able to see who's story is checking out and who is slacking. The PM could ask the member what are some things they feel they could improve on either for the next project, or for the remainder of the project. Ask the member is they feel there was anyone in the group that was not contributing as much as everyone else. Could go as far as having each group member rate one another, including the PM. Lastly (thanks BRIAN ;P), ask each individual member if they felt that their skills had been utilized to the fullest extent.