1. Condition. Just like you were going to buy a car or house or somesuch. Ask yourself, does it look like the previous owner took care of it? If not, there are usually more problems to be had and not worth your trouble. Dust and dirt and fingerprints and scratches etc. If all of the above are present and accounted for, just walk away. Cleaning a record can get rid of dust and dirt, but not scratches or the damage causes by mishandling or misplaying. Do you think that the person who gets fingerprints all on his vinyl is going to be taking special care with his stylus and turntable setup?
There no magic to it, although turning the album to the light, like you do with the cds, will help to see some scratches. The type of scratch does matter as the playing surface is a series of grooves, deeper scratches are trouble. They actually cut into the groove walls and can cause skips and jumps or unplayability. Light scratches or scuffs can be merely cosmetic.
2. Generally, the newer, heavy pressings are good quality. The labels are spending more on quality vinyl, and more of it, in a 180 or 200 gram release (usually even 2 discs) and they are being produced in far fewer numbers than in the past and sold to a niche, collector market that they Better be good. This type of vinyl is no longer just being mass produced by the hundreds of thousands for some pop bands that the average consumer won't remember in two weeks. Of course, this reflects in the high end prices on alot of these items. In the 70s and 80s, so many albums were being produced that shortcuts were taken. The vinyl pressing were super thin and many labels used recycled vinyl (i.e. scraps that got remelted) that even a brand new album tended to sound crummy.
Yes, like most things, a case by case judgement is called for. Most of those answers can be found online. (like whether a particular pressing is mastered well, or a better sounding press than the original, etc.)
3. Yes, some of the newer vinyl releases are even being pressed using the master from the cd. Here is a really good article that sorts through the variations.
Mastering Vinyl | How to make vinyl records and what it takes to mix and master for vinyl music.
Is it "worth" it to buy vinyl over cds is a very personal decision. Some people rave about the sound quality, some about the resale value, and some about the size and heft and art quality or the experience that having to play an album gives you. For most people, they just want to hear something and the easiest way to do that is the default. For a while that was cds and now they can just go online. Albums create a commitment (size, storage, time, care) that most people do not care about or have the time for.
Feel free to send me a note if you have any more questions.
Mark
These answers matched your original post which seems to have been recently changed.