1) There are multiple things to look after when proofreading subtitles. If you can’t manage them all in one read, then you’d better do a cyclic proofreading. Focus on one or two criteria at a time, proofread the whole translation by those, and then repeat the process until you have exhausted all the criteria.
2) In case someone else did the translation, you should watch the whole video first, in order to get a feel for what it is about, what terms are used, etc.
3) Rather than simply reading the translated subtitles in a text file, using a software that allows you to see the subtitles on the video and edit them where needed is the best way to proofread a translation.
4) Check the fidelity of the translation to its original. For that, you’ll need to go through the video and listen to it line by line as you do your proofreading.
5) Check for spelling, lexical and grammar errors.
6) Check if the translation sounds natural and is comprehensible to the native speakers of that language. Try pretending you don’t know English and can’t hear what the speaker says, and read the translated lines as if that’s all you have. And see what you understand from them. This can be done as a separate step where you mute the video and simply read the subtitles from start to finish.
7) Check if you can read each subtitle in the time it is displayed on the screen. If you can’t get to the end of a subtitle by the time it disappears from the screen, then you have two options:
a) try to rephrase that line in the shortest way possible
b) you can try extending as much as possible the time in which that subtitle is displayed on the screen.
8) Checking the time codes is also part of the proofreading process. Sometimes errors in the time codes can make a subtitle not appear at all on the screen, or appear for a shorter amount of time than it should. So, make sure that there are no delays in displaying the subtitles, that each subtitle appears and disappears at the right time, and that all the subtitles get to be displayed.