In positive terms, right speech means speaking in ways that are trustworthy, harmonious, comforting, and worth taking to heart. – Thanissaro Bhikkhu
The last of the four guidelines the Buddha offered about speaking wisely addresses empty words and their opposite, words worth taking to heart. Useless speech is a vast category with fuzzy boundaries, so maybe it’s easier to start with considering what kind of talk is worth taking to heart.
At the center would be the teachings of the Buddha, which cover a pretty broad field of precepts and practices and stories. The unifying principle is that they lead to awakening, not to sedation or denial or superficial enchantment. I think the useful question is: does this conversation shed light, or bring about wholesome states like reflection, compassion, understanding and wisdom?
One huge mistake most of us make in general conversation is to assume that our feelings form the most interesting, most pressing topic available. I’m not saying it’s never appropriate to say what we feel, but it might be appropriate to first consider whether we are speaking for any other purpose than to hear ourselves. So there’s another question: what is the purpose of my talking?
A lot of daily conversation is ordinary and repetitive, maybe particularly at work or school, where we’re meant to be focusing on a single set of tasks. This might be categorized as neutral speech, neither shedding particular light nor clouding reality over. In (morally) neutral speech, we might pay attention to whether we can develop clarity or gentleness, or improve our words in some other way. Sincere kindness would be a welcome addition to any interaction.