What happens if you sweat pipe copper pipe without flux?
There are several possibilities to consider. 


If you had really cleaned the joint beforehand with emery cloth or steel wool (both parts of the mated joint) there there is a good chance of a decent joint. The cleaning action helps clear out any oxidation on the parts that prevents the solder from adhering to the copper. (Keep in mind though that the simple act of applying heat to the joint also brings on oxidation.
If you had happened to be using a rosin core silver solder that flux built into the solder wire may have been more than enough to clean the joint for proper soldering.
An improperly cleaned joint may appear to not leak because it is tight or a small amount of solder may have flowed into the joint but it is no substitute for a properly made joint. One way to understand the sweat process success is to carefully observe how the solder sucks into the joint just as the silver solder wire is touched to the joint and melts. 


So if your situation is like #3 (and possibly #1) you will really probably want to take things back apart and re-do this work. Last thing you want is for the joint to come apart under mechanical stress on the pipe or start leaking after a year of so of thermal cycling of the joint. 

It can be really be worth while to get a handfull of copper fittings and a length of copper tubing and practice a dozen or more test joints to get the techniques down. (Note that standing water in or on a pipe near where you are soldering can be a huge detriment to getting good sweat joints.