1. Visit the school!
Make sure to visit each school.
For grad schools in the USA, there is usually an "open house" for prospective students where the schools invite you to visit for a couple of days.
At the open house, they give you
Lots of tours...
Lots of talks...
If they are organized, they set up meetings for you with professors and groups you have expressed interest in working with.
There are also opportunities, both in a professional as well as in a social setting, to interact with the current graduate students there. More on this in a bit!
There might be a research/poster symposium where different research groups showcase their work to attract new students.
You might be taken to different departmental events (we take prospectives to our daily "Astro coffee".)
They might show off their world famous football stadium, or new gym with a thousand swimming pools, or their newly renovated, gigantic library.
Take it all in.
Observe, listen, ask, learn.
Through activities at the open house, you will start to get the picture of what it might be like to be a graduate student in the department and in the school in question.
It will seem like there are a million factors involved in the decision-making process but go with your gut.
For each program, consider the people and the culture.
Which visit left you feeling the most comfortable with the idea of spending several years working in that department?
Which town/city would you be OK with calling home during those years?
Don't just focus on the work you will be doing, focus on the life you will be living.
2. Engage with the current graduate students.
Then, chat with them and learn about their experience.
3. Show your interest in specific research groups.
Examples of deal-breakers:
--Group has no funding for you.
--Professor makes sexist comments. Imagine spending YEARS studying under their supervision if you can't spend a day without feeling uncomfortable around them.
--Group's culture is clearly unsuitable for you.
4. Ask your questions whenever you get a chance.
Whether it is during the visit, or in a follow-up email to a professor, admin or graduate student --
You should ask any questions you have about what you will be doing:
classes, teaching, and research
Make sure you understand what the expectations are regarding each of these three things.
And, very importantly, ask questions regarding things that will affect you and your life every day, such as:
Where do the graduate students recommend staying? What is the housing scene like?
How much do they pay for rent?
What do people do for fun at this place?
What is the weather like? If you hate cold weather, it is still not too late to back out of a place that gets 8 months of winter.
Do they care about issues that are important to you?
This could be something like increasing diversity and representation.
Also, ask questions regarding success rates:
How many years do students spend in the program on average?
Figure out what questions you'd really like to get answers to and write them down!
Seek out the answers to these questions, and that should help you discover which school will suit you best.
All the best!! Let me know in the comments how it goes!