Please remember that there are no exact rules in nature. You may be "lucky" and have several issues at once. These are general rules to hopefully help you figure out the majority of pest issues. Also remember that a healthy plant is a lot less likely to get sick.
1. Something Ate My Flower
If your plant is missing pieces, something ate it.
Exceptions - hail, wind damage, fungal attacks can cause leaf and stem tissue to die quickly.
2. Strange Spots
If your plant has weird spots on it, likely a fungus among us.
Exceptions - hail and wind damage can cause leaf tissue to become abnormal. Insect feeding injury can cause weird spots especially insects that feed on the "juices" of the plant.
3. Leaf Issues
If your plant has curled, distorted leaves, it is likely herbicide damage.
Exceptions - leaf distortion can also be caused by insect feeding or using the leaves as protection/camouflage.
So Now What?!
If you have problem number one you likely need an insecticide.
If you have problem number two, you likely need a fungicide.
If you have problem number three, there's no hope for you except thinking that maybe it will grow out of it.
Need More Help?
If you are still not sure then swing by with some good photos or a plant sample in a sealed bag for us to help you determine what might be going on. We're always up for a little plant detective work.