How is 16s rRNA used to identify unknown bacteria?
It is not conserved, its sequence is not the same in all bacteria. Indeed, it has a slow rate of evolution (mutation) that make it perfect to build phylogenetic trees.

As correctly suggested by David in his comment, here are some references about the 16s rRNA and on how it is used to identify known and unknown bacteria.

http://www.sciencedirect.com/science/article/pii/S1369527499800526

http://nar.oxfordjournals.org/content/38/22/e203

https://www.ncbi.nlm.nih.gov/pubmed/18828852

https://www.ncbi.nlm.nih.gov/pubmed/27388368

https://www.ncbi.nlm.nih.gov/pubmed/27659439

https://www.ncbi.nlm.nih.gov/pubmed/18353221

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2760787/