How can you determine R and S conventions?

Answer 1

You have to use the Prelog-Cahn-Ingold rules.

The basic idea is that you give each atom attached to the stereocenter a priority, based on its atomic weight. If you have 2 carbon chains, you count the next molecule over, so:
C-O would beat C-C because oxygen has a higher atomic weight.

Then, with the lowest priority molecule facing away from you, often a hydrogen, you look at the orientation of 1-3. If it is clockwise, it's R. If it is counterclockwise, it's S.

Answer 2

niuchemist is absolutely correct. If you need a good visualization, any organic chem book will give pictoral instructions. If you need to do this for homework, don't be afraid to ask the prof or TA. They will be able to help you and possibly provide extra problems to practice on to make sure you have it down for the test.

Answer 3

Here try this link:

By the way its configurations not conventions ;)

http://neon.chem.uidaho.edu/~chem275/Images%20-%20Chem%20275/R,S.pdf#search='r%20and%20s%20configurations'

Hope this helped!