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.
If it does use the same frequenzy as a wifi card it should be possible depending on your card, skills, and willingness to invest time ;). It might be easier and quicker to go with something else though, look at the path Tim took to make his diy-spy for instance.
Using a device like a wifi spy should answer if it is there or not, Tim made his own diy-spy based on a cheeper device and has published the source code he used. Basing this device on a wifi card should be possible if it supports digital RSSI reporting or not. Starting of at this code it should be possible to find the signal you are intereseted in and working with that.
Another "simple" way to find out what frequency your keyboard uses would be to check it using a Frequency Counter (either borrow from a hackerspace, a friend, a ham or buy one [there are cheap options on for instance ebay]). The reason I say this is that some [I have seen] uses 2.4 GHz, and some 27MHz; and I would be highly impressed if you are able to read 27MHz using a hacked wifi driver ;)
You might wish to check out KeyKerki
This opensource hardware and software project enables every person to
verify the security level of their own keyboard transmissions, and/or
demonstrate the sniffing attacks (for educational purpose only).
They have done both 27MHz and 2.4GHz but this is a stand alone solution, not using a computer card.
To say "50MHz oscilloscope" is to say that it'll show signals accurately up to that frequency, but beyond that signals will be attenuated or not visible. This is a soft transition. No scope's electronics is perfect.
As with audio amps, the upper frequency limit is often defined where a sine wave input diminishes by 3dB, that is, loses half its power, compared to lower frequencies, when going from input to display. Read the scope's specs carefully to be sure.
Note that for complex signals or signals with sharp edges such as sawtooth waves, square waves, I2C signals, bluetooth signals, whatever, the upper frequency limit applies to the Fourier spectrum of the signal. Sharp transitions get mushy. The higher the upper frequency limit, the less mushy. A square wave right at 50MHz may appear so rounded off, but probably not quite as smooth as a sine wave.
For sampling rate, if the scope's amplifier and display are good up to 50MHz, and it's not an analog scope (dusty, old, vacuum tubes, ah good ol' days), the signal needs to be sampled at something like 4x or 5x that frequency to actually show a sine (or sine-ish) wave.
Nyquist says something about 2x, but think about it - if you sample a fast sine wave at 2x its frequency, you could by chance be sampling it once as it crosses zero on the way up, and again as it crosses zero going down. Then you'd see flat zero. So sampling is more than 2x. Nyquist still applies, but in having the response of the scope drop greatly before half the sample frequency. For your 50MHz / 250MHz example, there's probably a fast drop in response somewhere between 100MHz and 120MHz. Beyond that range, you won't see anything.