One thing nobody mentions is that some standalone sci-fi gets rebranded as a series opener years later when it sells well - so even if the book itself wraps up cleanly, you might find yourself disappointed if a sequel suddenly gets greenlit and marketed as "Book One." Beyond checking official publisher info, it helps to look at when the book actually came out versus when any sequels were announced; if there's a big gap, the original was genuinely meant to stand alone.
That said, books from the last couple years are probably safer since there's less time for surprise sequels to materialize, and you've got recent releases like *Project Hail Mary* or *The Kaiju Preservation Society* that wrap up their stories without dangling threads - worth filtering by "standalone" on reading sites and trusting reviews that specifically mention having a real ending.