The best test is to stick your finger into the soil as deep as it will go. You can use a gloved hand if you don’t like getting your hands dirty. Be careful not to damage plant roots. You should try to get down over six inches because plant roots hang out between six and twelve inches deep. If your finger comes up dry, you probably need to water. If not, you can wait. Another indicator is wilting. Sometimes wilting means you need to water sooner. Keep in mind, however, that wilting can have many causes. There's a good chance that wilting isn't due to lack of water. It's always best to use the finger test to be sure. There are gadgets that will test soil moisture, but none of them are as good as the finger test.