I have done some traveling and I have often stayed in hostels. These are great for many reasons. One of them is meeting people that share the same mindset and that actually need to sleep at night :)
My experience tells me that you can basically find 2 types of hostels (generalizing). Travel hostels, where most people are travelers and party hostels where most people are there for exploring the night life, drinking and have a place to sleep (if needed :). It's probably not the owners desire to choose a style but it often happens that these places start slowly shifting towards one of these (and of course they are not mutually exclusive, plus travelers also like to party).
From what I've seen this mostly depends on the city (there are clearly party cities where most people are there for drinking and partying all night long), but also hostel location. Reviews often help but they can be biased, too short or ambiguous (eg.: great place; great staff; loved to stay here; had the best time; etc.)
Besides the mentioned aspects what signs should I look, online, to tell if an hostel is a "party hostel" or a "travelers hostel"?