You haven't stated what type of work these employees perform for your clients, but it sounds like they provide some form of service in the field.
When it comes to team performance assessment, it definitely helps to use some of the methods highlighted by others, including routine customer surveys, random (unannounced) field checks, 360 feedback, and self-assessments. But I would go further and try to establish performance metrics that you score on a regular basis (i.e., once a month). An example of such a metric is # client cases resolved in a month.
You can use the metrics to evaluate the performance of both the larger team and individual team members, and compare performance of team members against each other. The right mix of metrics should allow you to identify top and low performers, your team's strengths and weaknesses, and identify problem areas that you need to address quickly. Good metrics usually follow what people refer to as the SMART criteria: Specific, Measurable, Achievable, Relevant and Time-bound.
Other examples that may apply to your case:
- Number of client visits in a month
- Time to resolution or handle time
- Number of client cases that remain open (backlog)
- First response time
Just try to come up with metrics that matter to your business and clients, make sure to score them on a regular basis, and take corrective actions to address problem areas or weaknesses as necessary.