1. At first check the ranking of the hostel in the valid traveling websites.
Like zhavroad.com, booking.com, hostel world.com and trip advisor.
2. Read the comments of travelers that were in the hostels as much as you can.
Checking their comments can give you a chance to know about how useful that hostel is for your persona.
3. Check the services that hostel can give them to you, maybe they are not enough or useful for you.
As hostels are not equipped like hotels you have to check to be sure it won`t make a problem for you.
4. One of the most important things about hostels is direction, check it to be sure about comfortable accessibility to the places you want to go.
5. If you have valuable things, check that hostel rooms have safety boxes or not.
6. Be sure about all the attendants that you need to talk to can speak English.
Some of the hostel`s receptions also can talk in other languages.
7. Make sure about all the facilities that hostel can service to you.
8. If you are smoking, check about smoking rooms of the hostels.
9. If you like to travel by tour, most of the hostels have many services in this case. Chose the one which is the best for you.
10. One of the most important things that you should care about it is how much time you have to refund your reservation and how much money they will return to you.