A method to test if oil is hot enough, is to hold a piece of wood into it and see if bubbles form around the wood. This is typically a good method for when there's a good layer of oil in the pan or when frying.
You may also drop a droplet of water into the pan and see how violently it reacts. This sounds a bit ambiguous I know, but when a strong sizzle occurs, you know it's hot enough. But please, don't use this method if you're cooking on fire, only for induction/ceramic.
Otherwise, I strongly recommend you to buy a simple thermometer to test the temperature. Let me explain why: It's very unlikely that your oil is evaporating. It depends on the type of oil you use by typically oil will start to vape around 230+ Celsius, and when this happens you are creating unhealthy situations. You would want to prevent that, hence why a simple cheap thermometer may be wise.