On my German keyboard it's Alt-Shift-7 (since Shift-7 is the forward slash).
Go to System Preferences -> Keyboard and check the "Show Keyboard and Character Viewer in menu bar" checkbox. Then click on the little flag in the upper right hand side of the menu bar and choose "Show Keyboard Viewer". Now you can try the effects of holding down one or more modifier keys to see which characters they'd produce.
In the Japanese ISO 646 encoding (a 7-bit code based on ASCII), the code point 0x5C that would be used for backslash in ASCII is instead rendered as a yen mark (￥), while in Korean encoding, it is drawn as a won currency symbol. Computer programs still treat the code as a backslash in these environments, causing confusion. Due to extensive use of the backslash code to represent the yen mark in text, some Unicode fonts like MS Mincho still render the backslash character as a ￥, so the Unicode characters 00A5 (￥) and 005C (\) look identical when these fonts are selected.