My answer might be coming a bit late for you, but hopefully it helps others in future:
Is there a way to know if my PR is invalid as of now (some website etc)? This will be helpful so I can plan my vacation accordingly.
No there isn't. You are considered a PR until the CIC is able to determine otherwise. This usually happens when
1) you try to enter and if the CBSA officer at the port of entry notices your prolonged absence and realizes that you probably won't be able to meet RO. Then s/he may report you to the CIC and CIC may start removal of the PR process.
2) you try to apply for immigration benefit from CIC for you or others linked to your application. CIC may do a check on whether your PR is valid or not in this case.
In either case, if CIC initiates PR removal process, you are still allowed to appeal this and if you can prove that you had a genuine compassionate reason not to move to Canada, you may even win the appeal (Living and working in the US ain't gonna cut it). Also note that anecdotal evidence suggests that in some cases CBSA officers only issue a warning and let people enter. But this is more likely if you are just under the 2 years you need to live there, and not for most of your 5 year period.
If the adjudicator determines that I'm no longer a PR at border / PoE (port of entry), would I immediately know that I'm reported for not fulfilling the RO? Do I lose my PR status immediately or after certain days (if so, how many days)?
They will most likely inform you, if they do this. It might take a while for them to process this. In this case they may or may not tell you to remain in Canada till removal is completed. Most likely CIC will offer you to voluntarily renounce your PR (much easier on them and much cleaner for you in the future), rather than forcefully removing you.
Is it a bad remark on my profile if I'm reported for not fulfilling the RO? If so, is it better that I myself renounce PR (from US, or at the border)? Would it affect my chances of getting a TRV in future?
Not fulfilling RO is not considered an offense (unlike Misrepresentation which is a big no no for CIC). So no, in theory future applications should not be affected.