With American pens, if it says "Made in USA" it's 1930's or later. That doesn't necessarily mean it's bad, and because it doesn't say that doesn't necessarily mean it's good. With Esterbrooks, I do feel safe in generalizing that any of them without the Made in USA will be good quality. The really early ones ("R. Esterbrook & Co's" or any with no "R" just "Esterbrook & Co" or " Esterbrook & CO" are VERY good. The & Co's date to 1890 - 1922 or so. The "No-R" Esterbrook impressions are before that date.
You can also look at the grind on a nib and that can give you some idea of the care taken with it. If the grind is carefully done, like it might follow the lines of the side slits, or if it has a double grind (both perpendicular and parallel to the slit on different parts of the nib) it's most likely a nicely-finished nib.
Some brands are better known for solid quality control, like Gillott, Esterbrook, Turner & Harrison, Birmingham Pen Co., Perry, Spencerian (which I believe was made by Perry), and some others. Some had more "bargain" sub-brands like Esterbrook's Penesco line. Turner and Harrison had their standard line, marked Turner & Harrison, which was consistently good quality, as well as their premier line, Leon Issac's Glucinum Pens, which are always of very good quality.
So, to sum up, for American Pens, no Made in USA usually means an older (and better) pen (except Hunt and Eagle often put "USA" on their pens even before the 1930's, so it can be confusing with them). A carefully hand-ground pen (as opposed to a stamped, or messy grind) almost always means a good quality pen. A double ground pen ALWAYS means a very good quality pen. (but many very good quality stub nibs didn't have grinds, though some did even though it has no impact on a stub nib, it was still seen as a mark of quality). If you're looking for Esterbrooks, "& Co's" and "No-R" imprints are the best, "& Co" without Made in USA is still good, with "Made in USA" can still be good, "Made in USA" with stamped grooves instead of a grind should be avoided, though they will still often be more consistently good than some modern pens.
My two-cents worth