How do doctors decide if you are a candidate for botox and dermal fillers?

By a complete exam. You doctor will evaluate you, the lines and wrinkles you seek to treat, and will recommend a treatment plan tailored to your needs. Generally speaking, Botox and Dysport work on lines caused by muscle contraction, while fillers treat creases and folds in the skin which exist in the absence of muscle contraction. Seek care from a board certified facial plastic or plastic surgeon or dermatologist.
Depends upon you. Depends upon what you are trying to achieve. Botox treats dynamic wrinkles such as the forehead lines, crows feet or the lines between your eyebrows, whereas fillers treat contour lines - such as the lines between your nose and mouth and mouth and chin. Sometimes the two may be supplemental.
They are different. Botox relaxes the muscles under the skin so that the skin looks less wrinkled. Botox does nothing to the skin itself. Severe wrinkles will not go away with a single treatment of botox, but multiple treatments let the skin remodel. Fillers are meant to fix loss of volumen and contour issues, not fix wrinkles. Flat cheeks and deep smile lines get better with fillers. Your doc can help you decide.
Muscle vs volume . Botox is used to relax muscles which will help reduce the appearance of lines during animation. Dermal fillers are used to plump lines and more importantly area to help contour the face back to more youthful appearance.
Things to consider. The dr takes into consideration how your skin looks how much fat your face is lost and how many wrinkles you have. They also consider your age and then decide how much Botox and filler would help the appearance of your face.
Based on hearing... What bothers you, and doing a physical exam. Botox works well for the wrinkles caused by muscle action, especially in the upper third of the face (forehead, frown lines, crow's feet). Filler injections fill in deep wrinkles, like the nasolabial crease, or plump up lip volume.