How do you check how much current a Mac USB port can deliver?

Solution 1:

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)

Solution 2:

Simply open About this Mac -> More Info... and go to USB on the left side of System Profiler. You will get current (now power or voltage) in milliAmpere but I guess this is what you need to know. E.g.

USB Receiver:

  Product ID:       0xc52b
  Vendor ID:        0x046d  (Logitech Inc.)
  Version:          12.00
  Speed:            Up to 12 Mb/sec
  Manufacturer:     Logitech
  Location ID:      0xfa130000 / 6
  Current Available (mA):   500
  Current Required (mA):    98