Log into Menchies.com, scroll down to "join now", then on top of the screen there is a button "check balance" . Type in the number on the back of the card. Check "I ma not a robot and hit the "submit" button.
Generally, any card's available amount can be checked by a cashier. Enjoy.
There is probably a number you can call on the back
Thanks! Your answer is awaiting moderation.