You can achieve this in two distinct ways:
1) use a reference (I'll give an example below)
2) measure other quantities that can be converted into an acceleration.
Here are some possibilities:
1a) Gravity. At rest, the accelerometer should measure the local g.
2a) Put it on a rotating platform. Knowing the rotational speed and distance to the pivot, you can calculate the acceleration.
2b) Put in on a swing
2c) Shoot it with a catapult
2d) Put it on a subwoofer
2e) ...
The list goes on and on. Practical methods depend on the type of accelerometer.