It says in my textbook ' after hybridisation with VNTR probe, the autoradiogram gives many bands of different sizes '. I tried looking on the net and it said there that probe is ' a radioactively labeled ssdna which binds to complementary dna ' My question is, how do we know that if the DNA we are testing does contain complementary sequences?
You should look up VNTR. Take a look at the wiki page and it will tell you that VNTRs are Variable Number Tandem Repeats, the length of which varies between individuals, making these irreplaceable in DNA fingerprinting and such.
The trick is, all individuals will have it, but the pattern will be different, so you probe such regions which are formed as a stretch of k-mers. And you already know that they will be complementary, because we all carry them.
If you know what genome you are looking at, and your probes are validated for that species, then your sample will have that sequence.