Is there a way to find out whether my GDK application was launched due to voice command or using a tap ?
Nope, that's not part of the API as of XE12.
It's being tracked by issue #426 in the Glass platform issue tracker. If you star that issue, you'll be notified when it's updated. Also, please comment on the issue to share your use case. Use cases help the engineering team prioritize enhancements.