As per this post the problem you experienced is a bug:
https://forums.whonix.org/t/tor-browser-download-issues-due-to-connection-interruption/2626/17
It can be very difficult to tell.
https://whonix.org/wiki/Warning#Man-in-the-middle_Attacks
https://whonix.org/wiki/Dev/TimeSync#Attacks
https://whonix.org/wiki/FAQ#Compromise_Indicators
Explained in the first link.