both engines are b18. the GS is a b18b, the GS-R is a b18c. you can check the stamp that is on the block. it should be right next to the header.

both GS and GS-R can come in 4 door models. all US GS-R come in 5 speed manuals.

the most accurate way to tell is by looking at the VIN like RB said. or you can look at the intake manifold. the GSR intake manifold curves like a lowercase "n" shape. while the GS intake manifold curves like a "U" shape.

then there is always the signs of vtec. like the obvious writing on the valve cover.