Explain methods of identification of transformed and non transformed cells.
Answers
It could be difficult to know if you were just using a random bacteria isolated from nature — especially since there are likely to be many thousands of different plasmids (1730 were present in a sequence database as of 2009). We could sequence all the DNA inside the bacteria, but that is still a lot of work ...
However this doesn't matter as much as you might think.
For example, assume we are using a plasmid that contains a marker (selectable gene) encoding resistance to ampicillin. All we need to know is that the bacteria were are transforming are not already resistant to ampicillin. This is easy to test — we just try growing the bacteria in the presence of ampicillin, if they don't then we can use our plasmid.
In practice microbiologists have domesticated strains of bacteria (a favorite is Escherichia coli — often abbreviated to E. coli) that have been studied for decades. In almost all cases you would be using one of these well characterized strains and so would not need to worry about whether there were unknown plasmids.