How do I tell string of pearls, string of bananas and string of tears apart?

All three are Senecio — now often placed in Curio — and they are frequently mislabelled at point of sale, so the plant tag is not reliable evidence.

String of pearls, Curio rowleyanus, has near-spherical leaves with a small pointed tip and a narrow translucent stripe running down one side. That stripe is a window that lets light into the interior of the leaf, and it is the single most reliable feature. Beads are roughly six to eight millimetres across and spaced fairly widely along a thin stem.

String of bananas, Curio radicans, has elongated, curved leaves shaped like a crescent, pointed at both ends, typically two to three centimetres long. The stems are noticeably more vigorous and the whole plant is more forgiving indoors — it tolerates lower light and less careful watering than string of pearls, which is why it is often the better choice for a beginner.

String of tears, Curio herreanus, sits between them: leaves are oval or teardrop-shaped rather than round or crescent, usually with visible longitudinal stripes rather than a single window, and often faintly purple at the tip in strong light.

Care is nearly identical across the three. All want bright light including some direct sun, a gritty mix, and thorough watering only once the medium is completely dry. All three rot quickly in a dense compost and all three are toxic to cats and dogs if eaten.

One practical note: these plants root along the stem where it touches soil, so the fastest way to fill a sparse pot is to coil a trailing strand back onto the surface and pin it down rather than taking cuttings.