What matters to you in a series - are you into action, story, character development, or more just something to watch casually in the background?
That's important because you can't really say objectively whether a series is "good." But you can check beforehand whether it matches what you're looking for. Watch trailers, read reviews on platforms like IMDb or Rotten Tomatoes - not just the average rating, but detailed reviews from people who like similar series to yours. Pay attention to specific criticisms: Does it say it has a slow start? That the humor doesn't land? That the characters are annoying? That tells you way more than a generic star rating. YouTube also has tons of channels where people seriously discuss series without spoiling anything - check those out before you start.
Another trick is to watch the first 10 - 15 minutes and then decide. Not a whole first episode, just really the opening - that's when you quickly figure out if the look, pace, and tone appeal to you. Sure, some series need a few episodes to warm up, but honestly: if you have zero motivation to keep watching after 20 minutes, it probably won't get better. And remember that your taste can change - a series that other people love doesn't automatically have to be for you, and that's completely fine.