Discerning possible left-wing nationalists

How does one tell whether a socialist organisation is left-wing or anti-nationalist?
Most socialist organisations, even revolutionary ones like freedomroad.org, profess internationalism, which only means that they believe all nations should work together and oppose imperialism (a principle we're discussing in 'Socialist Governance'). As a result, they generally support the rights of self-determination for various oppressed peoples, e.g., Palestinians, Chicanos, Eritreans, &c. They rarely mention European races in this, due to the (not unreasonable) perception that we Europeans have historically been doing much more oppressing than being oppressed.
That's all fine and dandy. But how can one tell whether such a revolutionary socialist organisation is left-wing nationalist, cosmopolitan (which I have no problem with), or actively anti-nationalist in practise? Based only on statements like the one I summarised above, there's no real way of telling. Certainly if one approached the issue of left-wing nationalism through the topic of globalisation and its effect on all cultures, they might be more amenable, but they might also blindly shun us in the typical antifascist way. Does anyone know anything about this through experience?

In my experience, the vast preponderance of socialist and communist organizations in the global north are cosmopolitan, and decidedly opposed to any and all expressions of nationalism in First World nations. In the global south, however, left-wing nationalist views are routinely espoused by socialist and communist parties.

The general idea among cosmopolitan socialists is that nationalism is only acceptable when expressed within oppressed nationalities, and only insofar as it aids in proletarian revolution. Immediately thereafter, nationalist sentiments become "reactionary," in their opinion. Many, if not most, cosmopolitan socialist groups support a temporary phase of self-determination for nations in the global south, but staunchly oppose the notion of even enforcing border laws within First World nations.

This is pretty sad. Essentially it turns the left-wing nationalist movements into propaganda tools so that the socialists of the global north can be seen to stick it to The Man. (I'm sure there are individuals who are sincere about supporting the oppressed, but if they are, they should become left-wing nationalists and be done.) Nick Cohen's What's Left? shows rather clearly the unfortunate results of such blind opposition to everything Western.

Not even enforcing border laws? What nonsense.

EDIT: Here's an example: the Freedom Road Socialist Organisation, a perfectly good group of revolutionary socialists, supports self-determination 'up to and including secession' for African-Americans, Chicanos, and Native Americans. One can only imagine what their response would be to someone who wanted self-determination up to and including secession for European-Americans, especially if the term 'white people' was used instead.