How to find out an Android Market app's ID

Simply go to the Android Market (Play) app in your browser (say the GMail app) like this

The GMail App in Google Play

and look at the URL

https://play.google.com/store/apps/details?id=com.google.android.gm#?t=W251bGwsMSwxLDIxMiwiY29tLmdvb2dsZS5hbmRyb2lkLmdtIl0.

The highlighted section contains the application identifier.