How can I best determine if a masters program is of high quality? I'm about to start my master in mathematics next year. I'm having a lot of trouble determining how some of the options available to me compare. They are all not very wel known programs, and I don't think it's a good idea to name them here explicitly so I won't. 

This whole choice is really stressing me out a bit. I want to choose the best one, but I have no idea how best to compare them. Things that I'm currently taking into consideration:


The number of enrolled students. This varies from only 10 to around 35 per year. 10 seems very low to me. And remember we're not talking about very prestigious programs here.
The structure of the program. Some programs make you choose a track in which you specialize, while others don't. You can of course still build that specialisation into your program on your own, but I feel that this explicit specialisation is probably better. I also think that better profs are attracted to programs where they can teach just their specialisation.


I would appreciate some input on the relevance of those two factors, as well as other things I should consider and ways of approaching this.

I will be studying pure mathematics and hope to be able to stay in acedemia if that is relevant for this situation.
There are several things you could try


Read student reviews online - sites like GradCafe offer
interesting discussions that will give you an idea how well the programs are run and how much faculty members are willing to go the extra mile to help their students get jobs after they graduate.
Look at USNews ranking as a loose reference. In my opinion (as a mathematician) those rankings do not fully reflect how good an institution is, but a department ranked #1 is quite likely to be better than one ranked #100. Bear in mind that US News won't give you the full picture, though, you could run into a fantastic professor at an institution that only gets a decent ranking.
Speak to the academic advisor in your department


Last but not least, your choice very much depends if you want to stay in the academia. If you're going into industry, reputation and the alumni network might be factors to consider. Otherwise, browse through the faculty directories and see if there's any potential match of research interests. For example, if you want to learn analysis, you're better off going to UCLA than to Harvard.

Good luck!