Well cnidocyte, usually people like to measure their intelligence by comparing it to other people.. This is widely known as an IQ test, which stands for Intelligence Quotient. This simply means what I just said, it is not a value of intelligence, but rather a measure of how you compare to others who have taken the test.
There are many free IQ tests online, but beware, they are usually not very accurate. I have taken several in one day just to check this, and the scores I got differed by around 30 points, which is significant. But if you find one that seems more or less legitimate, I recommend checking it out, even if just for fun.
But just IQ alone can not determine what you are asking..
The word "retarded" literally means "slow" or "lagging" (as I understand).. So is that how you are using the word? Meaning, slow of mind?
My suggestion is this: First, check for yourself your ability to understand, try to read difficult articles or textbooks, or some scientific or philosophical journals.. If you can understand even the most weirdly written stuff: that would be a good sign
Then, check how quickly you think on your feet, ask a trusted friend or family member to help you. Do random tests, for example: Ask them to throw in a "curve ball" in a conversation.. something like: You - "That was a good movie
", Them - "Just kidding, I hated it, why'd you like it at all?", You - (taken aback by this) "*insert your response here*".. see what I mean by thinking on your feet? There could be other stuff too like: Them - "really? you like that band too? What's your favourite song by them having (name here) in the background of the music video?".. could be other unexpected things that make you think on your feet, like random math questions like: Them - "Hmm, the meal cost $15.95, whats the 15% tip?", and if you can answer any of these and more fast enough, you will know that you are an average thinker in speed
But even if you prove to yourself that your mental abilities are not below average in understanding, memory or speed, there is more to check..
Check your emotions.. Do you have fairly good control over your emotions? Are they appropriate for certain situations, or are they out of place? How often are you emotional and how often are you calm and collected? What kinds of emotions do you most often experience? Are your emotions similar to those around you or not (there are ways to tell)? etc etc..
Naturally the biggest question here is, even if you check and test all these things, and everything is fine, then still, why do people think you are mildly retarded?
Several ways I can suggest testing this:
1 - When you are alone, look in the mirror and pretend you are looking at a person you just met, don't speak, just look. Act as you would when you meet someone new... try (and I know it's hard) to imagine that is not you at all, to forget that it is just a reflection, use your imagination... Then after a moment think to yourself.. What would I think of this person if I saw him for the first time?...... The answer might surprise you, if you pay attention and forget completely that it is you
2 - Pay close attention to what goes through your head when you meet new people, and I mean REally close attention, still have your conversation and do what you normally do, but pay attention to what you are thinking.... Not only that though, but also pay attention to what you are feeling.... And what you think of the person, and what you think they think of you, and what you think of yourself, and what you think they think about themselves.. While you two are talking......... Turns out that thoughts and feelings come out in an obvious way through the tone of voice, facial expressions and body language. You may not be aware of it, but your subconsciousness will tell you the truth, just pay attention to how you feel about the encounter.... If you need practice, simply learn to pay attention to how you feel toward things, but don't let feeling steer your actions, let them be simply advisers, but not rulers
I had more to say, but I have a tendency of leaving enormously long posts, so I will stop here.... I hope what I said will help, if not, perhaps there are other sources out there. If there are any problems still, you can post, and I will try to help when I see your post (tho I am not on here every day).