how do i tell if my mirage is a 1.5 or 1.8L and what is the best cold air intake that i should buy

Year?
Also, don't waste your money on changing the intake. It will make no measurable difference in power. It will just sound like it does. Just get a new air filter if the one in there is funky.

2000

According to the FSM, the engine model number is stamped at the front side of the top edge of the cylinder block. 4G15 = 1.5L 4G93 = 1.8L
HTH