= Polymerase chain reaction
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= PCR
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This is an extremely widely used technique as of 2020 and much earlier.
If allows you to amplify "any" sequence of choice (TODO length limitations) between a start and end sequences of interest which you synthesize.
If the sequence of interest is present, it gets amplified exponentially, and you end up with a bunch of DNA at the end.
You can then measure the DNA concentration based on simple light refraction methods to see if there is a lot of DNA or not in the post-processed sample.
Even <Ciro Santilli> had some contact with it at: <oxford nanopore river bacteria>{full}, see: <oxford nanopore river bacteria/PCR>{child}!
One common problem that happens with PCR if you don't design your primers right is: https://en.wikipedia.org/wiki/Primer_dimer