How can I tell if this is authentic Yves St Laurent?

[Vagabond]

It's a gold tone 21" Snake chain necklace. The drop tag has a vertical YSL stamped on it.

 

[Bron357]

Costume jewellery is notoriously hard to buy in the pre loved market because it’s not made of precious metals or jewels and so it’s very easy for those who make a business out of creating replica pieces to make believable replica pieces.
With vintage high end costume jewellery the money is in the name and scarcity not the materials. I’ve seen Chanel necklaces sell for thousands while their intrinsic value is like $50 if that.
The only way to be sure is to educate yourself by selling and handling authentic pieces. You learn to appreciate the smaller details like clasp type, quality of plating, chain size and finish etc.
But to answer your question, I’m not convinced that piece is authentic vintage YSL. The quality of the imprint seems poor, but if the price is around $50 it probably doesn’t matter if you like the look.