Yeah, the first (and most famous) studies were done by Ian Deary in the 80's and 90's, where he would have subjects perform simple tasks (such as checking how quickly you can cover a light with your finger), and he found a correlation with reaction time, which grew stronger as the task became more complex. There are more recent, higher quality studies, but Deary did a lot of the early work and is who I'm most familiar with since his work was mentioned in the bell curve. Should be enough info for you to google it
The bell curve is not debunked, it basically follows the NYLS, which is a multi decade, highly respected study. The book is one of the most respected surveys of the field.
There are people who claim the whole field of intelligence research is “debunked”, but we give them the same level of respect as people who claim the moon landing has been “debunked”.