Many tools, such as md5summer, will make the comparison for you automatically. But if you calculated a md5 checksum and have a number string of hex digits and a comparison wasn't made, then sure, compare the two manually, it will be very obvious if you have a natch or not. Due o the nature of the hash calculation it is extremely unlikely that it will just be off by a digit or two.

If the md5 is wrong then the file is corrupt. If it matches the file is good. Unfortunately, before the torrents became available I had about 50% bad files from the mirrors (and never had other bad downloads except Knoppix). So the md5 test is pretty important and useful.