Accelerometers are sensitive to the difference between the linear acceleration of the sensor and the local gravitational field. The data sheet for any accelerometer will denote the positive x, y, and z axes on the sensor package and, by convention, these are defined so that a linear acceleration aligned in the direction of these axes will give a positive accelerometer output.
A gravitational field component aligned along the same axes directions will, however, result in a negative reading on the accelerometer.
Using an accelerometer, you can calculate the angle between the gravitational vector measured by the accelerometer and the initial orientation with the gravitational field pointing downwards along the z-axis.
You can find excellent documentation explaining how to measure the tilt angle using accelerometers in the application note AN3461 that you can find in the following link: http://cache.freescale.com/files/sensors/doc/app_note/AN3461.pdf
Inclination sensing is a static measurement that uses the gravity vector and its projection on the axes of the accelerometer to determine the tilt angle. Therefore, to achieve the highest resolution of a tilt measurement, low-g/high-sensitivity accelerometers are required.
For example, the output of our 14-bit accelerometer operating in the ±2g range will change by 8192 counts as the accelerometer is rotated 180° from pointing downward to upward, giving an average sensitivity of one count per 0.022° change in tilt.
I hope this answers your question.