Learn Hardcore Russian: Я проверил текст сообщения и убедился, что всё правильно. - I checked the text of the message and made sure everything was correct.

Russian verbs come in pairs of imperfective (ongoing/habitual) and perfective (completed) aspect.
• проверял (imperfective) would mean “I was checking” or “I used to check.”
• проверил (perfective) means “I checked” – the action is finished.
Since the speaker completed the check, the perfective проверил is appropriate.

Why is текст in the accusative case, and how can I tell?

текст is the direct object of the verb проверил, and in Russian direct objects generally take the accusative case.
• For masculine inanimate nouns like текст, the accusative form looks identical to the nominative, so you don’t see an ending change.
• You know it’s accusative because it answers the question “what did you check?” (что проверил? – текст).

Why is сообщения in the genitive case?

Here сообщения is a genitive attribute modifying текст. In Russian the “of” relationship between two nouns is shown by putting the second noun in the genitive.
• текст сообщения literally means “text of message.”
• If you translated it word-for-word into English you’d say “text of the message,” so Russian marks сообщения as genitive singular.

Why is there a comma before что?