on H4 visa with Canadian PR

Re 1:
They do not need to figure out how many days you spent in Canada. If you are asked, and sooner or later you are likely to be (at the least, at the time of applying for PR card renewal), you must truthfully disclose all days outside Canada and a material failure (as in not just a few days or even weeks off here and there, but a substantial deviation from the truth) to do so can result in criminal charges, proceedings affecting your status, and even inadmissibility to Canada.

If you are outside Canada a great deal, it is a good idea for you to maintain a precise log of all border crossings. Since you landed just 2 years ago, you have another three years in which to accumulate 730 days presence in Canada to meet the residency requirement.

I suspect that sooner or later you will have to elect which country you want to stay in permanently.

As for what information CIC has access to and what is shared with the U.S. -- the minimum is that CBSA has "travel records" (how to obtain a copy of these has been posted around here somewhere, I don't know the topic title of the thread where though; see CBSA website) which will contain some record of a person's entry dates into Canada. This is probably far from complete . . . recognizing, however, that car license plates are usually scanned even if one's ID is not when crossing the border, so there should be a significant albeit incomplete record of the dates your car entered Canada. (Entry records are primarily derived from ID scans these days, and the vehicle license plate scans, not from passport stamps -- though the latter will be examined on occasion, such as in an app for citizenship.)

The rub: over the course of the last three to four years the amount of information collected at POEs and stored in readily accessible databases has increased dramatically (two or three years ago the U.S. and Canada spent many many millions of dollars upgrading the technology for this). It is unclear how much information is shared between the U.S. (BPS I think) and Canada's CBSA (CBSA and CIC have concurrent access to all their internal databases), but it is clear that a lot more information is available to them than is publicly disclosed. Moreover, there are many ways for them to put together the pieces of the puzzle to figure out such things . . . though of course the question there is whether or not anyone is motivated to do so. Usually not for anyone carrying a valid PR card . . . until it comes time to renew the PR card or one goes to apply for citizenship.

If and when the question of how much time one has been in Canada arises, there are many things which can raise a flag: driving a U.S. registered vehicle for example, absence of Canadian taxation, other evidence of residing outside Canada or at least not residing in Canada. And so on.

Bottom line: again, one's obligation is to report truthfully. No doubt there are many who do not, many who fudge, others who engage in outright deception, and get away with it, but it is fool's gamble given the consequences and risks of being caught.