How can I differentiate between a pimple and herpes on my lip?

Appearance. Herpes/cold sores are clear fluid filled blisters, usually in a small cluster, that are preceded by tingling or burning a day or two prior to the outbreak. A pimple is solitary, and is usually filled with yellow fluid, not clear.
Herpes or a pimple. Usually, not always, a tingling soreness type of pain occurs for hours to a day before the herpes virus comes to the surface and forms tiny blisters on a red bump. Pimples usually start red and do not develop blisters. Without treating or manipulating the herpes, the blisters scab. Pimples don't scab on their own unless they're squeezed. Don't touch the herpes to avoid infecting your finger.
How to tell. Herpes or cold sores are usually painful and water-filled blisters, affecting the edges of the lips (usually). Acne is red or red with yellow top, and usually affects the forehead, cheeks and chin. See a dermatologist if you are unsure.