If you start seeing that your child's general quality of life is starting to diminish, that may be a sign. For example, maybe they're not able to let go of things. They may not enjoy the things they used to. They might not want to do things or try things. They may just want to stay at home. They might spend too much time on homework and become resistant to any suggestion that they back off. If you limit homework time and see a major reaction, then you may need some help.
Anxiety can look like many different things. Sometimes children look like they're being defiant, but it's really because when they feel like they're at their limit and they just can't manage that discomfort, so they burst. They may become upset or be overreact. They can even become aggressive.
The key is to find what's going on beneath the surface. If you're concerned, talk with your pediatrician. They can be a great first step. Then, if they feel it's something that's a concern, they'll refer you to a therapist or a psychologist.