How did earlier chemist know they had discovered a new chemical element?

Question

I’m sure there’s a highly technical way of doing this today, but how did chemists in the 18th century know that what they had produced was a pure and uknown chemical element, and not just another chemical compound?

In: 4

Answer 1

Water was thought to be an element till late 18th century (1783), when they combined hydrogen and oxygen and got water, proving it as a compound.

So, they didn’t, till they could prove otherwise.