The four-different-groups rule
List every group attached to the carbon. A H counts. A methyl counts. The rest of the molecule on each side counts as a separate group — and you have to compare them. If two of the four groups are identical (two methyls, two H's, two identical chains), the carbon is not chiral.
Watch ring carbons carefully
A carbon in a ring has two "sides" of the ring as substituents. Those count as different groups if traversing the ring clockwise gives a different connectivity than counter-clockwise. A 4-substituted cyclohexane carbon (bearing the substituent) is not chiral because both ring paths are identical — but the C bearing a substituent on a 3-substituted cyclohexanone IS chiral because the two paths differ.
Carbons that look chiral but aren't
A CH₂ never is — it has two hydrogens, automatically two identical groups. A symmetric tetra-substituted carbon (like quaternary spiro centers with mirror symmetry) often isn't. Meso compounds have stereocenters but are achiral overall because an internal mirror plane cancels them out.
Beyond carbon
Nitrogen with three different substituents and a lone pair is technically chiral, but the lone pair inverts so quickly at room temperature that the molecule racemizes — you usually can't isolate single enantiomers. Phosphorus stereocenters are stable. Sulfur in sulfoxides is stable too.
Draw this on the whiteboard
Open the OChem Board whiteboard — benzene rings, wedge/dash bonds, and a clickable periodic table built in. No account needed.