With Taiwan Mobile, call *867# to check your current balance.
This page on TaiwanMobile.com has info about how to get info about your account, but it doesn’t reveal the *867# trick.
Today was the first full day of my second trip to Taiwan. We went to the Taipei Toy Festival 2011, the Shilin Night Market, and a good Japanese restaurant near the Yomi Hotel, where we’re staying for 3 nights.