Do an internet search on the name of the company, the kind of tour or even the actual tour you are looking at, the country or part of the country you want to travel and all other details that might be important to you.
Do not search for all at once, add more details when you have seen the first few general reviews.
And do not forget to check the reviews for the age, kind of person and activity level of those that have written them.
You will get other reviews from my parents than from me for the same trip, while my niece will want different things again. Go more with people who have the same kind of expectations as you will have and are in your age range or activity level.
My friend with poor mobility will look at reviews from people her parents age, due to her problems.
If you can get any worth of mouth reviews from people you know or have met, all the better. You can ask what they liked and why they liked it.
Do not be afraid to talk with strangers in a train or a tour group having coffee where you sit to ask which company they travel(ed) with and their experiences.