I don't know why OP says "the sentences are not interrogative". Obviously they are, regardless of whether the question mark is present or not.
1: How could you know if you have cancer?
2: How can you know if you have cancer?
OP has taken his two examples from titles of articles, in which context there's no possibility of them having different meanings. They'll both be followed by text answering the "title" question by setting out possible ways to self-diagnose that you have cancer.
In other contexts it would be possible to distinguish different implications. For example, interpreting could as the past tense of can in #1 leads to...
1a: How was it possible for you to know you have cancer? (perhaps implying disbelief or surprise)
...or one could see could there as equivalent to would, indicating an "irrealis" context. Using could/would like that "distances" the speaker from his enquiry ("I don't actually think I've got cancer - but if I did, how could/would I find out for sure?")
must be/must have is used to show that we are sure that something is true and we have reasons for our belief.
He looks happy, he must be hearing good news
Note that be requires an adjective or noun to follow it, not a verb, so we use a kind of adjective called an active participle hearing. Together with be, this forms a present continuous.
He looks happy, he must have heard good news
Have must be followed by a past participle or by been and a present participle. In this case, the past participle is added, and have heard makes present perfect simple.
In these examples, we state that the reason for our belief is that he looks happy now, but we would need additional information to decide which example is appropriate. The first would be appropriate is he didn't look happy a minute ago but now somebody is talking to him. The second would be appropriate if nobody is talking to him right now but, for example, he has just come out of the boss's office after his salary review.
We cannot directly build a future for this example, as he does not yet know that he will receive good news and so he probably does not look happy, and so we cannot base our belief on the way he looks.
Here is a different example where we can infer a future event from current information:
The blackbirds are singing: it must be going to rain.
Best Answer
The first one is a grammatical question. The second one reads more like a title than a question; it could be used as a header for a paragraph.
So, as an essay, you might see it like this:
or:
In a dialog, we might start with the question form: