There are many choices in customized and ‘off the rack’ team assessments. The 4-D system by Charles Pellerin is very well-supported and well researched. There is a simple 25 question assessment in ‘Games Teams Play’ by Lisa Bendaly (McGraw-Hill Ryerson). The Internet provides many free assessment tools (e.g., Mind Tools, Team effectiveness assessment). Most are based on team members’ self assessments and scoring of a number of team attributes. In this era of metrics, you may wish to find statistically robust measurement tools.
However, I personally think that there is also value in teams developing their own self-assessment tool through a carefully guided process that includes going through an exercise to define what a great team and great teamwork mean to them. They can then develop a descriptive list of key behaviours that relate to such areas as trust, communications, responsibility, etc. and a scale for scoring.
Regardless of the tool you use, a single assessment yields only a snap shot taken at a moment in time. To get real value, report results promptly to team members, debrief and discuss them, create action plans and provide workshops on key areas for improvement. Repeat the assessment to measure progress and, once again, report the results. People need to know they are making progress, and so does your organization.