Most Citrine purchased online and in rock shops is not genuine citrine. I am not saying that sellers are lying to you; in fact it is so common that it has just become 'the norm'. Synthetic citrine is in fact amethyst that has been heated or irradiated. The yellow colour is often more intense, than natural citrine. Synthetic citrine is darker yellow/orange. It sometimes has a reddish hue to it.
Synthetic citrine is often labelled as 'Citrine Quartz' or 'Lemon Quartz'. If you are unsure just message or ask the seller. Any experienced seller will know and is unlikely to lie to you.
Natural Citrine Synthetic Citrine
I have sold both natural and synthetic citrine. My customers seem to prefer the synthetic citrine though. I used to find that customers would complain about the lack of colour with the natural citrine.
If you have any questions - please message me