You’ll just have to memorize them….
-ER spelling is pronounced -AIR as in [chair]
inherit, inheritance, inherent, whereby, thereby, whereas, error,
-ER spelling is pronounced -EAR as in [fear]
coherent, coherence, criteria
-AR spelling is pronounced -ER as in [her]
straightforward, research, similar, summary, supplementary
-AR spelling is pronounced -AIR as in [chair]
area, variables, primary, apparent, aware, welfare, military, preliminary, contrary, voluntary, guarantee, paradigm, arbitrary, clarify