SAP Partner evaluation is a step-by-step process. Once you shortlist the partners based on certain criteria such as your business needs, controls, etc., you can evaluate them in a lot of different ways. You can gather information from outside sources like the clients that partner is currently serving, or has served in the past. Once you are sure about the partner, you can invite him for a demo session, where you can assess him on different parameters such as industry and technical expertise, functional knowledge, expertise in project management, background check, ability to deliver the implementation on time, keeping the budget, having required tools and resources to fulfill the needs of the implementation, long-term support post implementation, future development roadmap, Total Cost of Ownership (TCO), including, license fees (including third party), implementation fees (training and consulting), support, hardware/software and maintenance fees, so on and so forth.
