How do I know which USB accessory is using too much power?

Try removing one at a time, and/or checking System Report>Hardware>USB...

you haven't told us what problems you are having or anything about what external devices are connected. If a device is using too much power, then it is likely not to function. Some deices tell you that they should b directly connected to a USB port in the computer. However, many of the will work if connected to a powered Usb bridge. I would never use a non-powered bridge. If you need more help you will need to be more explicit about what is happening.