What is the meaning of output of an accelerometer
When the sensor is horizontal (back of the PCB flat on a level table) with the package of the chip "up" (so you can read the printing on the chip) the Z axis should read about +1g and the X and Y should indicate close to zero depending on how close to horizontal the package is. 

The X and Y readings are proportional to the cosine of the angle to level. The Z reading is proportional to the sine of the angle to vertical.

It is not possible to distinguish gravity from acceleration (Equivalence Principle). 

Edit: An acceleration of 1g upward behaves the same as gravity downward. Imagine you're in an closed room floating in space, and then the room is accelerated upward at +9.8m/s^2, the force you feel inside the room would be the same as the gravity on earth.