Easiest way to check if a USB drive is bootable

Recommendations seem to be to use System Preferences -> Startup Disk

All I did was, after I ejected the USB drive, that I unplugged it and plugged it back in.

If it's bootable the iMac attempts to read it!

I was in the process of creating a bootable USB for Snow Leopard for my old Mac. Did the 'biz' to the USB drive and putting it back into the new iMac, it brought up the contents of the disk as if it was about to prepare to install!

So hope this works for you too.