English has become the lingua franca in many parts of the world, and in almost any part of the world, at least some minority of the population also speaks English. As a result, learning English is in demand, both within developing nations and among their citizens living in predominantly English-speaking countries. Any would-be teacher or student needs to be familiar with the nomenclature involved with teaching and learning English. Understanding the difference between ESL and EFL is a first step.

Understand the umbrella term. Both ESL (English as a Second Language) and EFL (English as a Foreign Language) fall under the broader category of English teaching known as TESOL (Teaching English to Speakers of Other Languages). This means that both systems of language teaching, although unique, share the fact that English is being taught to someone who does not yet speak the language.

If outside of an English-speaking country, it's probably EFL.
Look at the context. The easiest way to tell these two systems apart is to examine where the classes are being taught. Generally, if the classes are taking place in a non-English-speaking country, what is being observed is English as a Foreign Language. If the classes are for immigrants to the U.S. or U.K., then English as a Second Language is probably being used.

Observe the students. Generally, ESL takes place among multilingual and multinational students within an English-speaking country. The inverse is also true: EFL usually occurs among a homogeneous group of monolingual students in a non-English-speaking country.

Making basic purchases can often be intimidating for ESL learners.
Pay attention to learners' objectives. Often, people who are studying ESL want to learn how to survive in their second language. As a result, they will be much more interested in studying vocabulary and grammatical structures one would encounter in everyday life: going to the store, buying a car, ordering food, for example. EFL students, by contrast, are often studying the language as a school requirement, as a hobby, or in order to engage in international business.

Explore your methodology. ESL and EFL learners have different strengths and weaknesses. For example, the multicultural origins of ESL students can provide for lively discussion and diverse opinions, while EFL learners are responsive to the exotic nature of the English-language and its culture. Effective teaching methods capitalize on these strengths.