When bacteria grows, which
involves asexual reproduction,
several cellular changes take
place, such as growth the mass
of cells, an increase in
chromosomes and creation of
new cell walls, and the cells split
into 2, then 4, then 8 and so on.
Starting with a bacterial "broth,"
growth can be measured after
incubating a small amount of
that broth in a sterile medium
and counting the number of
bacterial colonies per volume.
Bacterial growth can be
measured by either considering
the changes in weight (which
involves centrifugation) or
directly counting the number of
cells or colonies. There are
several other methods; however,
they would involve the use of
expensive lab equipment that
you may or may not have. Some
counting methods are discussed
here.