A Kanchipuram saree is woven from mulberry silk in three parts — body, border and pallu — which are woven separately and joined. The join is called the korvai, and on a real one you can find it: a fine interlocked seam where two colours meet.
Hold the border and the body and pull gently in opposite directions. On a korvai saree the join holds and shows a faint zigzag. On a printed or power-loom imitation there is no join at all, because the border was never separate.
Zari is the metallic thread in the border and pallu. Traditionally it is silver wire wrapped around a silk core and then gilded. Tested zari carries a mark. Cheaper sarees use a coated polyester that looks similar when new and does not age the same way.
Weight tells you a great deal. A genuine Kanchipuram with real zari is heavy — noticeably so in the hand. That weight is silk and metal, and it is most of what you are paying for.
Stored properly, folded in unbleached muslin and refolded every few months so the creases move, one of these will comfortably outlive the person who bought it. That is rather the point.