Most collectors classify their cans by country, but sometimes it's not so easy to tell where a specimen comes from.

Three main parameters should be considered:

the country where the beer was BREWED;
the country where the beer was CANNED;
the country where the can was originally SOLD.

Almost every can clearly indicates the town and/or the country of the brewer, and in most cases also the canning site is stated.
Where the can was sold may be told by the size and type of the specimen (see sizes of cans and types of cans); European cans produced as of 1980 usually have a bar code, called EAN, in which the first two numbers indicate the country of origin (scroll down the page or follow this link for a table of european barcode numbers, or EAN).

Fortunately, in most cases all three operations (brewing, canning and sale) are carried out in one same country, and this makes it very easy to tell where the can comes from.

In a number of cases, though, points 1. and 2. are the same country, but point 3. is different: for example, there are many Canadian brands specifically canned for the Japanese market, or New Zealand export brands for Pacific and South-West Asian countries, etc.
In such cases, the can is usually listed as belonging to the country of origin (i.e. Canada or New Zealand).
Other recent examples are some Skol cans for the Brazilian market which have been produced both in Brazil (by one brewing company) and in the USA (by another brewing company): both versions might be collected, because even though they are identical, one should be classified as Brazilian, and the other one as a USA can.

A more tricky situation is that of a brand brewed in one country, then taken to another country for being canned and sold.
Several British supermarket companies (Tesco, Safeway, etc.) sell "Dutch export lager" quality: the beer is brewed in the Netherlands - as stated on the can's side - but the size (often 440 ml.) and the supermarket brand tell us that these cans will only be sold in Britain.
Most collectors consider them British cans, some others consider them Dutch.
The same situation concerns the only German Democratic Republic (DDR) brand ever sold in cans, Wernes Grüner, whose beer was canned and distributed only in France: most collectors consider them French (not East German).

An even more complicated situation is that of Karlovacko and Nektar brands from Croatia: probably due to the delicate political situation in the former Yugoslavia area, there were no canning lines (or they had been shut down), so that the beer was brewed in Croatia, taken to an Italian canning line, and then brought back for the Croatian market. These cans appear officially in the Italian catalogue, but many collectors consider them Croatian, because they were only sold in Croatia. Recently, the Karlovacko brand has changed canning line, and these cans are now produced in Austria.

In such cases, each collector is free to consider the specimen as belonging to one country or the other. It is important, though, to be aware of the fact that the can was not produced in the same country where the beer was brewed.

Who produces cans?
Many new collectors will be surprised to know that breweries do not produce their cans.
Actually, they are made by companies who are specialized in metal works, tins and boxes for other canned food, etc.
These companies own several canning lines, and they often have different branches in different cities or even in different countries.
More than one brewer might choose the same company to have his cans made; this explains how, despite the relatively small number of canning companies, the number of beer cans (as well as coke & soft drinks ones) is really vast.
But for big breweries the opposite situation is also quite common, i.e. to have one same can produced by different canning companies.

Usually companies "sign" their products with a tiny mark, which is found on the side of the can in the lower part of the body (in older "steel" cans, often near the seam).
Some collectors give the small mark a great consideration, by keeping in their collections cans which have an identical label but different marks.
There are some cases in which these marks give useful information about the can; two examples:

the French company FerEmbal used to mark old "steels" with a logo and a serial number in which the first two digits state the year of production (i.e. Ferembal 72 N 8568 means that the can was made in 1972, etc.), which enables to date these specimens quite easily;

most brewers from the Netherlands have always chosen the Dutch branch of Thomassen & Dryver company (originally from Belgium), so that during the years 1970-1990 about 80% of the cans from the Netherlands had a "tD" mark, but still nowadays it is quite common. The mark has a progressive series number (usually 1 letter and 3 numbers), which is rather useful for identifying and classifying the many label changes, as each different one has its own corresponding code.