I simply want to ask my colleague that do you know cabbage? a green leafy vegetable. So what I should say Do you know cabbage? or Do you know cabbages?
"Do you know cabbage?" and "Do you know cabbages?" are both grammatically correct, for reasons that I will explain, and both mean the same thing. But neither of these sentences mean what you want them to mean.
The sentence "Do you know ____?" is generally used for people ("Do you know my friend Jim?") and facts or fields of study ("Do you know calculus?"). So the sentence "Do you know cabbage?" makes it sound like "cabbage" is a field of study. It is almost like asking "Do you have a lot of knowledge about cabbages?" or "Are you an expert on cabbages?" This question from the English Language and Usage Stack Exchange gives more details.
So if you asked your friend "Do you know cabbage?", then your friend could reply, "No, not very well." or "Yes, I grew up on a cabbage farm." or "What is cabbage?"
This is not exactly what you want to know. If I understand you correctly, you only want to know if your friend has the most basic knowledge about cabbages. So it would be better to ask:
Do you know about ____?
Do you know what ____ are?
Do you know what a ____ is?
Are you familiar with ____?
Have you heard of ____?
Now that we have the right questions, let's fill in the blanks.
For most nouns, you would always use the singular form for the third sentence and the plural form for all the other sentences.
Do you know about computers?
Do you know what computers are?
Do you know what a computer is?
Are you familiar with computers?
Have you heard of computers?
But "cabbage" is a special case. It is more flexible than most nouns. All of the following sentences are correct:
Do you know about cabbages?
Do you know about cabbage?
Do you know what cabbages are?
Do you know what a cabbage is?
Do you know what cabbage is?
Are you familiar with cabbages?
Are you familiar with cabbage?
Have you heard of cabbages?
Have you heard of cabbage?
"Cabbage" has more possible sentences than "computer" because the singular form "cabbage" can be used as either a countable noun or an uncountable noun.
If I said to you, "I have a cabbage" or "I have a lot of cabbages", that means I am talking about heads of cabbage. I am using the word "cabbage" as a countable noun, like "apple" or "computer". There is no unit of measure needed. One cabbage is one cabbage.
If I said instead, "I have some cabbage" or "I have a lot of cabbage", then the meaning of the word "cabbage" is slightly different. Perhaps I have a bag full of cabbage leaves, or a bowl full of shredded cabbages. I am using the word "cabbage" as an uncountable noun, like "bread" or "music". It is a raw material that must be divided into units (cups, bowls, bags, etc.) before it can be counted.
So, for example, if you asked me, "Do you know about cabbage?", I would hear "cabbage" as an uncountable noun, because that makes the sentence grammatically correct. This is why there are more correct sentences for "cabbage" than for "computer".