How can you identify optical isomers?

See this old answer, and also this one with respect to diastereomers.

Some practical tips, given a representation of a chiral centre:

The interchange of ANY 2 substituents at the chiral centre, the carbon nucleus, #Cl# for #Br#, #H# for #F# etc., results in the depiction of the enantiomer. And thus, given the stereoisomer on the left as we face the page, if we exchange #Cl# for #Br#, we should generate the optical isomer, i.e. the isomer on the RIGHT HAND side of the page. Interchange again (it does not have to be the original two substituents) and we generate the stereoisomer of a stereoisomer, in other words the ORIGINAL stereoisomer. Are you with me?

Some people find it very easy to vizualize representations of stereoisomers. I am not one of them, and would always require models to inform my reasoning. You should have a play with a set of molecular models in order to establish that what I have said here is kosher. Models are always allowed in organic chemistry tests (and in my experience, they tend to be underutilized). And, as I have mentioned before, you will always find a set of such models on the desks of distinguished professors of organic chemistry. The prof will have a fiddle with the models when an idea strikes him or her.