Without peer review, how do I know if a preprint is flawed?

You do not and you should be aware that it could be flawed. However, you should apply a similar wariness to journal publications since peer-review often misses important flaws in papers.

Preprint commentary by experts in the field, whether in the preprint server’s comment section, on a third-party review site, or on social media, can help to put findings into context.