by attaching them to a chip.
Probes are mechanically fixed to the chip and then they are also labelled with fluorescent tags. once the DNA complementary to the probe binds to it they fluorescence, which is in turn controlled by the number of probes attached on-chip and how much DNA binds to it.
A microarray is a fast and quick way of doing the genome-wide analysis.
following is an image which depicts various types of probes available in microarray analysis
a video to understand it better is given below: