[Animal1234]

I am having a hard time determining R/S when the lowest priority(usually Hydrogen) is in the plane.  Without using a model how can I figure out a way to identify if it is R or S.

Thank You

[Woofuls]

Practice is the best way. Check out this website: http://www2.chemistry.msu.edu/~reusch/VirtTxtJml/sterism3.htm

[orgopete]

While not universal, there are some easy tricks. For example, if the smallest group is toward the front, the configuration is the reverse. If the largest group is toward the rear (2,3,4), the configuration is the reverse. If the largest group is toward the front (2,3,4), the configuration is the reverse of the reverse. If neither the largest or smallest are to the front or the back, then a transformation must be done. The problem is this becomes more complicated than the mental juggling (which is what I do).

[Agathiyar]

This comes by practice, assign the priority of the atoms by 1,2,3 and 4.  Leave the smallest (most cases hydrogen), now rotate the numers (big to small),  if it is clockwise then R (but the smallest atom (H) should be inside the plane). If the rotation of the priorrity is anticlockwise then it is S (again the smallest atom or group should be inside the plane).  If the smallest group is upward then the sign changes into opposite.

BTW, I have problems with syn and anti, erythro and threo in vicinal cases, any help?   

[Agathiyar]

Sorry, I forgot to mention that if the Hydrogen is in the plane (your original question), you can do even number of shifts to make the molecule same. ie you can push the hydrogen inward and bring back the other group to the plane, this is one shift,  do another shift like this with other groups (not touching the hydrogen) then you end up with 2 shifts (even number of shifts). Now you have the same configuration as original and easy to assign R/S configuration.

[discodermolide]

I am having a hard time determining R/S when the lowest priority(usually Hydrogen) is in the plane.  Without using a model how can I figure out a way to identify if it is R or S.

Thank You

Read up on the Cahn-Ingold-Prelog rules, it's really quite easy.