The best thing to do is just take it all in. Make sure you go to a few, and then you can compare.
Before you go, be honest with yourself about your child and what kind of environment would work for them / what they may need in future (e.g. if suspected SEND, a strong SENDCO may be important). Have a think as well about what matters to you as a parent.
Make sure you look around and listen carefully. A close friend once mentioned that she had looked at a school where she noticed that they had no artwork on the walls the library was virtually non existent, a headteacher who kept talking about "discipline" and toys that didn't look played with at all. She was really put off as she didn't think that it showed them allowing kids to be kids.
In contrast, DD's school allowed us to look wherever we wanted, have a quick chat to the kids or teachers if we wanted, and the kids genuinely looked like they were having fun. The headteacher also made it clear that she had high expectations, but was very accessible to speak to and interested in the kids who went with their parents. An headteacher like this really sets the tone for the school.
It may be worth asking about how involved the PTA are. We have found that a good PTA makes a huge difference in both making the school a proper community, and also funding extras.
I always laughed when people said they just knew, but we genuinely did when we walked into DD's school. It was really only confirmed, though, when we visited other (perfectly fine, in most cases!) schools and felt flat!
Good luck :)