I bought turbotax software from amazon (downloaded mac version). Therefore, I know it is safe to use. However, if one wants to verify the software has not been tampered with how do we do that ?. Mac gives standard warning "Do you want to open software downloaded from internet"
Some download websites show the file hash of the installer on the website ... so then you are supposed to download the file , calculate the downloaded file hash , and compare to see if the hashes match to confirm the download wasn't tampered with.
Similarly if you have a known legit version of the software and a questionable copy of the same version of software you can get the file hash of each of the files and compare to ensure it they are the same and haven't been tampered with.
Note this only works if the 2 pieces of software are the same exact version because file hashed will change version to version.
Here is samples of how to get file hashed on a Mac