How to identify if popup download of Adobe flash Player was infact a virus or malicious on?
Unless you specifically went to Adobe &amp; downloaded it right from their page at https://get.adobe.com/flashplayer/ then assume it was malicious &amp; discard it.

If it arrived in the form of a Disk Image (.dmg) then so long as you don't mount the image, the file itself is inert &amp; can safely be trashed.
If it was a Windows executable (.exe) then that is also technically safe on a Mac - ie, it will not run - so can also be safely discarded.
If it was a compressed file (.zip, .7z, gz etc) then so long as it remains unextracted, again it is safe to simply discard.

If it was anything other than one of those types, or if you extracted any archive, then scan your computer with something like Malwarebytes or send the file to VirusTotal for further analysis.