It might be difficult to accurately gauge the strength of a letter of recommendation, because the content is highly subjective. Often it is recommended for a student to sit down with her recommender if her interactions with the recommender have been somewhat limited (because, for instance, you have been a little shy in the lab), but the person nonetheless agreed to write a rec on her behalf. But, most of the time, when one has the opportunity to sit down with a recommender, you would want to tell them about your plans for the future, as well as what relevant experience, credentials and skills you might have, hence the common advice to send your CV and personal statement to your recommenders.
But, at some point, you may ask about what schools you should apply to. Oh, of course, there are some clueless professors, including one of my research-based recommenders, who could trash some schools that are otherwise legitimate for a subfield, especially at the low end of the scale. (That recommender advised me against Brown, even if it was the final decision of the last student my supervisor wrote letters of recommendation to US schools for, and applying to WUSTL instead) However, if, in light of your discussion with the recommender, she recommends you to apply to schools that you’d otherwise consider to be out of your league, you are either dealing with a clueless recommender or the recommender actually thinks you will have a strong recommendation from her part.
Also, take in account that what’s considered “top-X percent”, as on some letter of recommendation cover sheets would say, where X is usually 5, 10, 20, 25 or 50, depends on the level of the student being vouched for. For example, a top-25% student as a senior in undergrad is not the same as a top-25% student in the final year of a 2-year masters. Depending on where the latter fits in the top-10%-to-to-25% interval, the funnel effect induced by the cycle shift from undergrad to masters means that the student could have been a top-10% undergraduate, since only the best ~50% will make it to graduate school in, say, BSc+MSc+PhD countries.
Undeniably, a more detailed letter is stronger than a generic letter with ~100 words of content, because one such letter could tell more about the student, so make sure the recommenders write comprehensive letters (although it can be quite hard to do if you waive the right to see the letter, which you should do because non-waived letters will be taken less seriously)
And, for the personal statement, it is recommended to review it with a recommender, yes, but also with someone outside your intended subfield. After all, admissions committees are made up of professors in a variety of subfields: it is important that your personal statement can be understood by people outside your subfield, so avoid using too much subfield-specific jargon.