Vantage electric guitars, when they are cheap, tend to be a pretty good deal. Their higher end guitars are not worth buying, but if you want to get the best you can for very little money, they are durable and reasonably playable.
As for acoustic guitars, if you dont know anything about the instrument, areasoably certain gauge of the instrument is if you turn it over and look at the back. If there is a line that runs down the middle from the neck to its bum, and the back of the guitar is made from two halves of the same piece of wood, it tells you that somebody made enough effort to make sure that the guitar will have a balanced tonality. It's a good sign, and it's easy to spot.
But be careful that it's not just one whole piece of wood and somebody has drawn a line down the middle to make it look like it is better quality than it is. Inspect both halves and make sure they are mirrors of each other.
This general rule applies to classical, folk, and dreadnought guitars.