Like all topics on art, there's going to be a lot of opinion in these answers. Here's mine.
Art critiquing typically contains elements of both subjective and objective criteria. It's hard to be completely objective when critiquing art as, ultimately, art is supposed to appeal to our subjective feelings.
The signs of a good art reviewer, to me, are those that are aware of the subjective parts of their review. As long as you are aware of your subjective opinion, you likely are also taking into consideration some of the more objective criteria as well.
Some things that I believe help one become a better art critic:
- deep interest and appreciation for the craft side of things. A terrible movie can still have exceptional workmanship in various areas. Being able to separate the craft from the final product is good.
- experience. With film, that's simply having seen a whole lot of films. This gives you a body of work to compare any other work against.
- broad understanding of the history of the medium.
- interest in all aspects of the process (costume design, lighting, sound, etc.)
- a willingness to explore all genres. Diversity.
- a devil's advocate approach. Try to come at the piece from many different angles. Try to understand other's POVs.
- avoid IMDB discussion boards at all cost (unless you're looking for good examples of really poor film critique)
If you're asking how do you, personally, decide a movie is good or not, that can be entirely subjective, as all that really matters is whether you liked it or not. I would argue, however, that the more you know about all of the above, the more you can appreciate films that you maybe personally wouldn't have initially liked. You could argue it's a bit like wine or whiskey. One's palate can become more sophisticated over time via experience and exposure.