how can I ensure a download is perfect

hi , being new to here, I've got some questions to ask because I come here. Hope someone would help.

Is there any way to ensure a file is perfectly and correctly downloaded from remote server / websites? (while checksum may not be available)

I find that quite some time the file being downloaded could stop unexpectedly before it's finished, although i am downloading with high-speed and reliable broadband. The 'stop' is usually not indicated / warned to the user under the Firefox or Chrome download status windows.

It is recommended to use IDM (or other 3rd party download manager) to solve this problem. However, it's like splitting up the files into parts might be likely to corrupt the files (according to the official FAQ of internet download manager)

Any ideas to solve the problems? At least, any programs will 'tell' the user if the download is stopped unexpectedly. As I said, Firefox and Chrome don't tell. For example, if a file was originally 100MB before it's downloaded, but it could stop at 80MB (this is not a problem, I can download again), but it doesn't indicate in the windows that the file is incomplete. (This is a big problem)