No signature file available for verifying apk
https://bitmask.net/en/install/signature-verification
Download signature file
The signature file for each download has the same name as the file but with .asc appended.
For a listing of the available signature files, see:
All Android files https://dl.bitmask.net/client/android/
However on the download page for the android client, there are no .asc signature files available, and no way that I can find to do a verification of an apk downloaded from the website.
On https://bitmask.net/en/install/android the two ways listed of downloading the app are from the website or from Google Play. If there is no way of verifying the apk from the website, then the only way of getting a verified apk is from the google app store which is not a good situation.