The answer to this question depends on how you would define "altruism".
Altruism is defined as the belief in or practice of disinterested and selfless concern for the well-being of others.
Imperialism is defined as a policy of extending a country's power and influence through diplomacy or military force.
By comparing both concepts' goals, it isn't difficult to conclude that they are counterproductive and contradictory. How can a nation claim to be altruistic when it is forcing its power and influence onto another entity?
Consider this saccharine-sweet and fictitious exchange:
Country 1-"Here, take this doughnut. It's delicious."
Country 2- "No, I don't want the doughnut. It's not my kind of thing."
Country 1-"Why ever not? It's delicious! You NEED this. You just don't know it yet!"
Country 2-"Thank you, but really, no --"
Country 1- *shoves a thousand doughnuts into Country 2's protesting mouth".
The word altruism possesses a positive connotation that clashes with what lies at the very core of imperialism.
Historians express that American Imperialism kicked off with the Spanish-American War of 1898.
To summarize, the U.S. won and Spain had to relinquish claims on Cuba. Also, the peace treaty demanded that Spain cede sovereignty over Guam, Puerto Rico, and the Philippines to the United States. Unfortunately, did Guam, Puerto Rico, and the Philippines have any say in the matter? No.
The U.S.A. - "Look! I've got this Treaty of Paris that states you are all free from Spain--"
Cuba, Guam, Puerto Rico, Philippines - "AWESOME!"
The U.S.A. - "Not so fast. You belong to me now. Now you can cheer. Praise me! Praise me!"
Cuba, Guam, Puerto Rico, Philippines --??? "Ehhhh?"
Cuba - "But what about my independence that you promised?"
The U.S.A. - "Oh, yeah. You're free to go..."
The Philippines - "This isn't right! We fought the Spaniards for freedom and now you're going to rule over us? How is this any better?" (Read to find out about the fight for independence from the U.S.A.!)
As expressed in the exchange above, the U.S.A did not fight Spain out of the kindness of its red, white, and blue heart for the well-being of others. Oh no, no, no. There was a quid-pro-quo outcome to be won. Primarily, the U.S.A would gain more industrial power, thus securing new markets to sell and produce its goods.
When one weighs the pros and cons of imperialism, it's hard to claim that the ends justify the means.
So, the second half of your question, "How do you know?" needs to be answered. I'm hoping that my quick and non-exhaustive response inspires you to answer that for yourself.
Happy researching and writing!