If you're booking online (eg hostelbookers, hostelworld) the confirmation email almost always tells you what time check-in is (and checkout).
However, I've found that usually, most popular hostels accommodate that you may be arriving on an early flight or train, and will let you leave your luggage there and come back when the dorm/room/bed is ready for your check-in. However, again, check their site - for example many of the YHAs in North America charge for the use of this room.
Generally, however, people leaving have to check-out some time between 10am and 12pm, depending on the hostel, then cleaning happens (some places insist you not be in the dorms during this time) and new arrivals can check in some time after this - often from 12pm, 2pm or 4pm, roughly. Again, it'll depend on the hostel, it's not a country-by-country rule.