How can I tell if a lower left control arm is OK?

If it is indeed cracked, it should be replaced and soon,it could cause an accident.If you have doubts, have some one take it off the vehicle and show you.thats the only way to be 100% sure

Did you take car in for a alignment issue? Or was it in a shop for rotine service and it was "oh by the way we noticed something else." A small dry crack in a bushing in itself is probaly normal for the age of the car. Unless you have a tire wear issue or pulls one way or the other when driving on the road I suspect the guys that said it was ok is correct.