Yes there is. They know when you enter and leave Canada. As long as they see that you have spent 2 years there out of 5 you are ok. But it is always good to save extra supporting documentation such as boarding passes, utility bills, pay stubs, tax returns, etc.
You say you will live in Canada for 2 years and then return at the end of the first 5 years to renew PR. Be very careful. They count the 2 years (730 days) TO THE DAY. Any day you enter Canada is included, but any day you leave Canada is NOT included (unless you return on the same day). And also, the 5-year periods are not discrete, they are rolling. A new 5-year period starts every day. So for example:
1 January 2015: You land and become a PR.
1 January 2017: You leave Canada and return to HK. In these two years you did not leave Canada for even one day.
31 December 2019: You return to Canada to renew PR.
1 January 2020: You leave Canada again to return to HK.
In the above scenario, on 31 December 2019 you have met your residency requirement and are eligible to renew PR. But starting 3 January 2020, you will be OUT OF STATUS, even though you just renewed your PR card, because during the 5 year period from 3 January 2015 to 2 January 2020 you did not spend 730 days in Canada, and therefore you are subject to losing PR status! Or suppose that during those first 2 years you flew to the USA for a weekend holiday, or flew back to HK to visit your family for a week. Then on 31 December 2019 you will not have met your residency requirement and will not be able to renew your PR card. Again, they count it to the exact day!
The only way to make sure you are not at any risk of losing PR status is to ensure that there is NO 5-year period starting on ANY day where you were not in Canada for at least 730 days. Spending a 2-year chunk of time in Canada and then leaving for 3 years is NOT a good idea. You are likely to lose your PR.
For even more details you may wish to consult a Canadian immigration lawyer. Be careful!