There are many SEO tips & tricks websites and blogs out there.
How do we know which ones are updated?
How do we know if the tips there has been tested?
How do we know if the tips there are correct?
How do we know that the tips are still working (or ever worked)?

There is no one definite answer. The best practice is to keep yourself updated with the sources:

• Read what the search engines have to say! we personally read frequently blogs like GoogleWebmasterCentral. Don’t expect to see there links like ‘click here to download GoogleBot’, or ’our google algorithm is free and available to read here’. That will never happen! however there’s tons of important information there (though not always super updated), but hey, that’s Google - they will never mislead you and it’s always smart to read there what not to do rather what to do.
• Create a couple of mini blogs and experiment on them. Search engines send their spiders to your sites or blog about every two weeks - so apply the tips and wait for a while to see the changes.
• Read SEO blogs. This is our favorite. So which one to read? there is no one source that the big guys read, but it’s be smart if you check the ranking of that blog before you test. Use Google’s page rank
  (this is a nice free tool - here) or use Alexa’s traffic ranking here.
• One of our most favorite sources is a members only blog: SEOMOZ (since 1996). The blog is not a blog per se, it’s actually more sort of a quite expensive site (about $50 a month) but hey, this blog has great tools to analyze your website and blog and lots of articles which are available to download.

Good luck. if you know of good SEO tips blogs , feel free to comment here below and please provide the link.

How often does Google update its SERPs?

That is, how often does it incorporates the information that it gets from its crawling bots and applies it to the SERPs (Search Engine Results Pages)?

I think there are 2 possible cases here. On the first one Google would update the SERPs in real time and constantly, as soon as new information is retrieved from its crawling bots. For example, if a bot was crawling a certain page and discovered that the Title tag was changed, it would immediately report it back to a central data center, that would in turn update the database and the ranking positions for this particular website.

A second possibility is that Google has a fixed schedule to update all the results together. For example, it could do it once every 48 hours, and on every update it would combine all the data retrieved from crawling bots and recalculate all the SERPs simultaneously. The schedule here could be daily, every 48 hours, every week and so on.

This factor is quite important because it gives bloggers and webmasters a time frame to check the results of your optimization efforts. Suppose you discovered that all your h1 and h2 tags were messed up on your site. You learn how they should be properly used, fix the problem, and then start monitoring specific keywords to see if your ranking positions will be affected.

But how long should you wait to see the results? Let me know what you think!