When I first started playing golf, I thought my swing flaws were this unique combination of precious snow flakes that I could never fully understand or identify. It really wasn't until years later that I start to realize that these flaws -- and frankly the flaws of pretty much everyone -- are a lot more common than I once thought.
Full Disclosure: Here's the basic check list I go through when looking at someone's swing on a My Swing Thread.
1) I check their ability level (listed handicap). This is important because a low handicap player needs to be handled differently, and I think this order here can change. Generally, I don't try to assist really good players for obvious reasons, but this list might be fairly universal regardless.
2) A new one I've learned and sometimes need to remind myself of: I make sure the camera angle they've chosen is solid because I don't want that to play tricks on my eyes. This alone can throw off pretty much any advice you give someone.
3) It helps to know their stock ball flight, and their biggest miss, especially if they're a good player.
4) Grip, stance, posture. Despite what Stack and Tilt may tell you, setup is extremely important. I generally recommend the strong single action grip. Does this guy's grip stink? Is there an arch in his lower back? Where is his eye line pointing? How much are his feet flared out? How wide is his stance? What direction is his spine tilted? Towards the target, level, or away from the target? What's his ball position? How much shaft lean is he using? Where is the center of his hips located? These static positions have to be addressed before I recommend anything in regards to their actual motion.
5) The actual motion. Obviously this section has multiple sub-categories. But it doesn't take a genius to figure out that pretty much every bad player doesn't get his weight forward enough, moves his head around all over the place, flips at the ball, has a downswing path that is too steep, etc. Like I said at the beginning of this post, we aren't unique beautiful snowflakes and bad players tend to make the same mistakes, just as good players tend to do the same things right.
So while it does take practice in terms of picking what to say and how to say it, we all generally see the same things here again and again, which is probably one reason why @iacas and @mvmac have made multiple videos in the 5SK video section for both them and us to use as embedded links to better illustrate our suggestions. It's basically like picking which pill to prescribe at which time.
When it comes to any of this stuff, when I see something I think the person is doing wrong, I'll typically recall a post Erik or Mike has made in the past for a similar issue, I'll find that post, read what they wrote to confirm or disconfirm what I'm seeing, and then I'll do my best to parrot it while still putting my own spin on it. When I'm blatantly wrong, they tend to do a good job of finding my work and calling me out on it. Sure, sometimes my bad advice slips through the cracks because there are so many posts here to monitor, but usually, they do find them and fix them for the user. Then some good discussion usually ensues and we can all learn. Or, I get it right, and get a thumbs up or two or three.
I could keep going, but I doubt anyone would keep reading ... But this is the basic blueprint I go by when giving advice around here. It helps to read a lot, especially the posts the pros make because that obviously influences the way I go about things.
I think my best advice related posts are the ones that are the shortest and sweetest while still nailing down the problem and the fix. I know a flaw of mine is to get too wordy or to point out too many things, so I do my best to edit so I don't lose the reader (this post notwithstanding of course ).
I've attended several 5SK clinics (as well as other non-5SK clinics) so I really tend to contribute to threads where I've had the same flaw as the OP. Given my experience with the problem, I certainly have an advantage in helping over situations where I didn't have a certain problem and thus possibly don't fully understand what they're going through or will go through trying to fix it. I try to remember what and how I was told certain things and what it was like for me trying to change the picture.
Like @Zeph said above though, I all too often feel that the more I know, the more I don't know. I don't presume to know everything, and I make mistakes. But the more I read, contribute and partake in this forum, the less crappy I think I get at it.