Perhaps unsurprisingly given its name, Limetta lemons are sometimes green in color (via Citrus Variety Collection). Limetta lemons can also have very low acidity levels depending on which variety you get, so they are often also referred to as sweet lemons. If you're looking to distinguish a Limetta lemon from a "true" lemon like a Lisbon, focus on the leaves. The first will have leaves that are a bit oval with pointed ends, versus the latter which is primarily oblong.