I just noticed a freckle on the bottom of my foot that was not there before. How do I know if it is plantar melanoma?

See your doctor. While this might be something very benign, it is important that you see your doctor with something that is very new or changed in appearance. You might need a biopsy to determine what it is, and appropriate care started. Some skin lesions can get worse quickly (such as melanoma). Get it looked at!
It depends on the cl. The clinical features of this lesion would be helpful to determine its nature. If it is indeed a small freckle(less than 5mm) it could be watched for sometime and observed for any changes in size and color. A dermatologist should look at it to determine whether to monitor it for a few months or to proceed to remove it, if it is suspected to be a mole/melanoma. I believe we have time to monitor it.
See a dermatologist. A dermatologist will be able to tell you if it is a benign freckle, or if it needs to be biopsied. Don't delay, because if it does need to be biopsied, the sooner it is done, the better.
Get it checked. By your family doctor. If he/she is suspicious, you can be referred to a dermatologist for a biopsy.
ABC's. Although i agree with all the posts previous, use abcde's to help relieve some stress before you see a doctor (you still should make an appointment to be sure or download an app on your iphone that does online consults). Is the freckle "asymetrical"? Are "borders" irregular shaped? Is the "color" uniform or multicolor? What's the "diameter"? Also is it "elevated"?