The bread (or cracker etc.) trick is more accurate, but with practice you can get a pretty good idea by flicking in a few drops of water and seeing how it reacts.

If you drop a grain of rice in the oil and it floats and cooks it's around 350

I'm with Mr_Vittles. The spoon trick works for me, but more often I will use the tip of a wooden chopstick.

You can also tip the end of a wooden spoon in the oil, if it starts to bubble around the tip, then the oil is ready.

Nutcakes has it right. If you don't have a thermometer the bread method is really the best way. That trick is old and steeped in the history of culinary.

But first, what type of oil are you using? Some have lower "smoke" points than others. Usually you can tell when the surface starts to shimmy a bit.

when you think it is close, drop a cube of bread in. If it sizzles and browns in about that a minute you are good. If it burns in that time, it's too hot. But depending on what you are putting in, the food will cool the oil a bit at first.