Most sarees on the internet are not what they say they are. India has a Handloom Mark, and even a law reserving the word — but in practice nobody polices an online listing, and sellers have learned that the word sells. So if you want to know what you're actually buying, you have to learn to read the cloth yourself.
Three things tell you almost everything.
The first is weight. A genuine handloom is heavier than you expect, even before you add zari. Pure mulberry silk weighs differently than viscose silk. Khadi cotton has a density polyester does not. Pick the saree up. If it feels light for what it claims to be, ask why.
The second is the back of the pallu. On a real handloom Banarasi, the floral motifs on the front carry through to the back — that is how a real loom takes the design across the weft. On a power-loomed imitation, the back is flat and the threads are cut. This works for most weaves with motif work: turn the saree over.
The third is the blouse piece. A handloom saree is woven as one continuous piece — body, pallu, blouse — on the same loom. The blouse piece carries the same weave structure and the same zari thickness as the rest. If the blouse piece you are given looks printed, dyed, or fabric-stitched on, the saree is probably not what it claims to be.
There are other tests — the burn test for fibre, gold-purity for zari, count-thread for cotton — but most of them are impractical to do with a saree you have not yet bought. Weight, pallu reverse, and blouse piece are enough to filter out ninety percent of mislabelled stock.
What you cannot test before buying is whether the seller will tell you the truth when you ask. That part is on us, and on every other seller. Ask the question. If the answer is vague, walk.