When trying to take off, it must be on level ground, not the slope of a hill or the motors will wind up quickly. You must arm and quickly take off to knee high. Try to get it flying within 5 seconds of first giving it power. If you take too long, the motors will wind up (trying to make it level) and that is one cause of flipping on takeoff. Don't slowly increase the throttle till it lifts. Don't steer it while it is on the ground. Both of those contribute to flips. Right after arming a small blip of throttle and then instantly back to zero will show you that it does take off pretty much straight up. For that test, try for just enough power to get off the ground and drop back down.
It will either obviously flip quick and hard or it will take off basically straight up.
If you do the following, you assume some risk. Be careful. It is quicker than you are, especially if it is set up with something backwards. If it is also stronger than you are it could cut you anywhere. You can do this to get an understanding that even with the motors spinning up, it does work in flight. For small quads, you can put on a heavy jacket and gloves for some protection and grip it very tightly (It may try very hard to flip) hold it level and facing the same compass direction as when you armed it, and add the smallest amount of throttle. Grabbing it from the bottom works best for my quads, but watch out for props. If it does it's best to twist away from you with high power even with low throttle, you have something wrong, like wrong prop (left rotation or right rotation), wrong motor direction, ESCs plugged in wrong FC connectors or FC is not mounted with arrow top side and point forward (search rotate virtual in wiki).
If it doesn't try to twist away and you can feel it twisting to stay level, you can rotate to where it wants to be. Yaw is very weak, and you will probably have to spin your body to match the desired yaw (compass angle from when it was armed). This will tell you a lot. You have basically proven that the stabilization works.