How do you check how much current a Mac USB port can deliver?
The Knowledgebase article Apple Computers and Displays: Powering peripherals through USB contains some general guidelines and illustrates using System Profiler (System Information in Lion) to provide information about the output (according to specifications) of a Macs USB ports.



If you want to test the real world voltage output of a given USB port on an individual Macintosh at a given load you will have to use a test device. (USB Test Circuit)