All the jewellery that is solid, on the surface must leave the engraved mark of how many karats is that.For example, Solid silver jewellery has S925 mark ah the backside or side that is not so eye-catching location. (if it's pendant backside or left/right side, necklace generally at the tail/hook part, rings are inside the ring band, earrings at post/backside of the body.)
All our solid S925 designs have S925 mark 
Solid (rose)gold jewellery also have 8K, 14K, 16K 18K, 24K mark on the surface. If you can't find any of the marks it is not solid.
The plating/vermeil jewellery doesn't have this mark.
For really solid S925, Solid (rose)Gold won't attract by the magnet.
But S.Steel materials are attracted by magnets.