For GSM phone (AT&T, T-Mobile etc):
- Insert a sim card from a different carrier
- Turn on the HTC phone. If the phone is locked, it will prompt you with "Network is locked, please input unlock code"
For CDMA phone (Verizon, Sprint etc):
- Turn off the phone
- Hold down the Volume Down + Power keys. It will show the flash screen
- It should say "SPL-0.57 Cmonex" if the phone is unlocked