Good morning;
Usually it is pretty obvious what a "good" teleconverter will be. The main thing to look for is the way that it is constructed. The best designs seem to be of a seven (7) glass element construction. This also means that the glass parts will be a noticeably bigger diameter inside the teleconverter than the cheaper and less effective simpler designs.
Just do not ask too much of a teleconverter. The results you get from a 1.4 X model will usually be better than the results from a 2 X teleconverter by the same manufacturer. The 3 X models may be pushing it.
Please note that no teleconverter is designed to be used on a wide angle lens. Normal focal length lenses of about 50mm and longer only.
There are a few that I can recall. The Vivitar 2X Macro Focusing Teleconverter was available in a 7 element version. The Canon people had some good ones. The Minolta 200-L, 300-S, and 300-L 2x teleconverters where the "S" and the "L" refer to short and long focal length lenses with the dividing line being at about 200mm. The Minolta APO models are lens specific, but they are very good. There are a couple of the Nikon ones that are also very good, especially in their 1.4 x models. The Kino Precision Kiron MC-7 2X Teleconverter. The Quantaray 7 Element 2x Teleconverter. Is this enough to get you started?
In most cases, yes, you will lose some resolution, but you do gain from the optical image magnification. The main comparison to be made is the trade-off between what you gain from the magnification against what you lose in the resolution and contrast. Even with some of the simpler but well made cheaper ones, you do gain something. With most of the 7 element designs, you will gain overall. The main thing is the history of the individual teleconverter that you have and how it has been treated in the past.
All bets are off it it has been dropped, just as with any used lens you might consider. Then it becomes a question of finding a way to test that particular sample to see if it is something that you can use.
Enjoy;
Ralph
Latte Land, Washington