You can use:
% get_files
If you want specific types, then use:
% get_files -filter {FILE_TYPE == Verilog}
If you really want to know what are the source files in your Source Set, then use:
% get_files -of_objects [get_filesets sources_1]
| AR# 47745 | |
|---|---|
| Date | 12/15/2012 |
| Status | Archive |
| Type | General Article |
| Tools | |