"Imitation silk" (polyester / viscose) looks similar but costs far less. A few tips to judge whether what you got is real mulberry silk.
Pull a thread and light it: real silk smells like burning hair, the ash crumbles into black powder when rubbed; polyester smells like plastic, forms a hard lump that won't crumble. Only take thread from a hidden spot, and confirm the sample can be damaged.
Real silk is cool and smooth, with a slight drag/resistance (a "gripping" feel), and a soft sheen that shifts with angle; imitation silk is more "slippery", with a harsh, flat shine. Clench and release: real silk wrinkles naturally and recovers slowly; polyester stays crisp and wrinkle-resistant.
Rub the fabric between your palms: real silk makes a faint "sha-sha" sound (scroop); synthetics are basically silent.
When wet, real silk's strength drops noticeably and it breaks easily; dry, it is tenacious. Imitation silk changes little when wet.
Real silk has a real cost; "silk" far below market price is probably fake. Look for the composition label 100% mulberry silk / Silk 100%. On our Quality Assurance page we publish tested metrics for qualified products.