How do I evaluate the legacy system application software?

Factor	Questions
Comprehension	How difficult is it to understand the source code? Is the source code visible?
Documentation	Is there any documentation available?
Information	Is there a data model for the system? Is data duplicated? Is the data used consistently?
Performance	Is the performance adequate?
Programming language	Is the programming language still in use? Are compilers available for the platform?
Configuration management	Are the components of the system managed by a configuration management system? Is there an explicit description of the components used in the system?
Test data	Does test data exist? Is there a record of tests and results?
Personnel skills	Are there only a limited number of people who understand the system?

Reference: Software Engineering, Sixth Edition, Ian Sommerville, Addison Wesley, ISBN 0-201-39815-X