The best way to tell what spells will work and which ones won't is to
Have a good grip on reality.
Understand the limits of magic and what it can and cannot do.
Those two things will really help you distinguish between the plausable and impossible spells. Besides that, many spells are written poorly and without much research behind them. You can always check the information in them elsewhere.
For instance, if a spell instructs you to use a green candle for wealth or healing, you can look up what green candles are used for and verify that they are used for one of those two things.
Also, you can learn to create your own spells so that you don't have to rely on the ones provided on this site. They would be more personal, and you could feel more confident about them.