"The Marguerite Casey Foundation Organizational Capacity Assessment Tool helps nonprofits identify capacity strengths and challenges and establish capacity building goals. It is primarily a diagnostic and learning tool. Results from the Assessment can also help grantmakers deepen their understanding of the current capacity of their grantees as well as track their growth in capacity over time." Format of the tool is an Excel file.
This webpage from the Institute for Human Services defines and explains in a simple way its view of organizational assessment. At the bottom of page, you can click on the link entitled "McKinsey-Ameen Capacity Assessment Sample Questions" (below graph) to open an offline survey. While you get no analysis of your results, the survey can still be used as a basic tool for you and your team to identify possible areas of improvement among the ones explored (leadership, management, operational, and adaptative capacities). Also available in French and Spanish.