You can't guarantee the technical quality of the video unless you worked on it, however there are some indicators.
These include the size of the files - as a rule of thumb the larger the file the better the quality (but there are lots of exceptions). One major one is the codec - As a rule of thumb, the newer the codec the smaller the file for the same quality.
Also for torrents the number of seeders and name of the group releasing the torrent are useful indicators of quality - more seeders implies its better quality - and less likely to be corrupt.
In order to verify the completeness of the download, most torrent sites have checksums for the complete download. Download a checksum calculator and compare the checksum of the file you have with the checksum specified.
Typically a torrent will work (but will skip or be blocky) even if some small parts have not been downloaded - make sure you have a decent player which can handle corruption.