If it did record images of your computer every 30 seconds, you would aventually find a huge directory and would know where it keeps stuff.
You leave your computer on overnight for 12 hours.
12*60=720
720*2=1440
1440*200=288000
That would be 288,000k if each image was 200k(288 megabites).
It's not hard to find a directory that big.
Secondly, you could monitor network access when your parents use the keylogger tool, and thus figuring out what it does.
If it was truely a keylogger, you could write a program that buffers 1 gigabyte of data to the buffer. It would be easy to find 1 file that is a gigabyte.
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