What I've found useful, and which worked reasonably well for teams which have been working over a period of time together is to collectively estimate complexity of tasks that must be completed.Say on a scale of 1-5. That gives you a reasonably agreeable scale of measurement (not a universal scale, but the one that the team accepts). As time progresses, you see how many such tasks are completed by any person in a given time frame.

Not that this is foolproof, or that it works right from the beginning of a project, but over a period of time, you tend to get a good sense of how productive each member is.

[Edit: Typo fixed]