If it's a new webcomic that's just starting out and only has a few pages available, I look at the posting timeframe on the dates of when they were posted, I read the premise (personally I like bizarre-o plots with farfetched ideas. Manga does this a lot and when it's executed correctly, it's worth it).
I usually focus on art less since a good story doesn't have to be about good art imo, it just has to have a an engaging plot with likable characters. At least for me, I know there are a lot of ppl where it's primarily the art that gets them interested first.
If it's a comic that's been out for a while but it's "new" to me since I just found it, I look at how many episodes are posted and if the creator posts them in a timely manner. If they haven't updated in a month with no explanation as to why, then I won't sub to it. And I also see how active that creator is, either in commenting back to their readers or checking out their links on their profile to see how active they are on their other platforms.
I know it sounds like I go crazy when I check out a new webcomic, but I don't have a lot of time to read. So if it's a comic I'm gonna be dedicating my scarce free time to, then I want to make sure it's gonna stick around for a while with characters I'm gonna enjoy reading about.