Hand-foot-and-mouth disease or just a simple oral ulcer/mouth sore. how can i tell?

Don't guess/get exam: You can't; get exam. Canker sores are common aphthous ulcers of mouth that occur situationally, often in response to stressors, are bacterial or viral, & can be treated topically with many otc products that reduce local discomfort & potentially speed healing. However, oral/dental problems & a number of systemic diseases cause canker sore-like lesions--crohn's, behcet's, std (sexually transmitted).

Ulcers: H. F. M. Disease usually presents on kids less than 5 years old. Oral ulcers in that disease are yellowish.