Well, the oscilloscope should have its own internal function generator for calibration purposes. Usually there will be two exposed pins on the front panel where the probe should be connected. It should be explained in the manual what type of waveform you should expect. This is also important for probe calibration too.
I don't know of any other easy way to calibrate it.
You could always mess with microcontrollers and try to generate some sort of calibration signal, but unless you have another oscilloscope to confirm that the device is producing correct signal, you can't be sure if the oscilloscope or the home made function generator or both are broken.
Another option would be to get it professionally calibrated, but that could be expensive.
The problem is that you are using a MEMS digital accelerometer, and what you are reading is the SCK (serial clock) pin of the serial interface. In order to function, that sensor needs to be interfaced with a microcontroller, that sets it for the sampling frequency, the range and so forth.
So you don't have to expect a square wave with 100Hz frequency, but a fast (depending on the bus bitrate) spike, corresponding to a transmission. Expanding the spike, if the scope is fast enough, you should then see the clock square wave inside the spike.
Moreover, if you don't set the SPI interface correctly, the uC will not generate the clock (the sensor operates in slave mode), and you won't read any value.
If you want to see a 100Hz signal, you could probe the Int pin, which sends an interrupt to the microcontroller every time a measure is available. Then, if you handle the interrupt from the microcontroller properly, you wil see the pulse corresponding to the transmission every 10 ms (100Hz).
But make sure that you're not using motion detection; in that case, only when an acceleration is measured, it will generate the interrupt.
To read the data at the SPI port, the simplest thing is to configure the communication with the sensor; otherwise, it won't send data at all. Then, check if the microcontroller is getting the interrupts and if it's reading the data the sensor gives; you can use a timer to add a timestamp to values and check the frequency they come.
(still WIP)
Best Answer
I've never damaged or heard of anyone damaging an oscilloscope.
If the scope has a 3-prong connector, don't connect the ground lead to something that isn't at Earth ground, or you'll blow some fuses and burn up the alligator clip.
If you're being stupid and floating the scope to measure something not at ground potential, don't leave an alligator clip hanging freely to brush against the chassis. :D
Don't connect it to anything greater than the voltage limit on the front of the scope (400 V).
Don't clip the probes to something and then let their weight + leverage bend and break the clip tip. I'd like to know of better ways to do this.