The problem is that there’s a lot of different things that you could call “intelligent” and, likely, you’re going to be dumb at some of them. For example:
- Able to memorize a lot of stuff
- Able to produce novel ideas
- Able to chop big problems up into smaller, easier problems
- Able to control oneself and not succumb to various biases, fallacies, shallow desires, etc.
- Able to manipulate and ingratiate yourself with others
IQ tests, for example, might largely be testing 2 and 3. And anyone who is good at both of those things would be called “intelligent”, regardless that they’re pretty crap at the other three categories.
Effectively, the issue is that it’s so hard to be above average across all metrics that just being at a high level on more than one is already quite spectacular. Nicola Tesla would be horrible at 4 and 5 and, averaging in this scores with 2 and 3, he goes from being a genius to being ordinary.
4, you can basically test yourself. If you wanted to gain a hundred pounds or lose a hundred pounds, could you? If news from every TV channel, streaming service, video, and visual medium started producing content of the quality of The History Channel, could you switch to text news or would you consume the least-bad visual variant that was available? If your spouse left you, could you quickly recover and move on with your life? Were you able to resist drugs and alcohol, despite all social pressure?
5, you can try cold calling people to sell them a time share. The more money you make doing that, the better you are at that style of intellect.
Most likely, testing everything, you’re average. So the smart answer is: Test yourself on your strong points and if you’ve got enough on those to lord yourself over others, then you should be satisfied that you’re intelligent. If you can’t but you’re also not in a home for the mentally disabled, haven’t been wiped out by vegas, and haven’t fallen for any scam emails, then you’re probably fairly average across the board.