You won't know for sure until you ask them, but if you notice they seem more withdrawn with other people, too, it might not be the cold shoulder at all. [substeps] If you notice a really severe personality change and other mutual friends do, too, talk to the person immediately.
[header] How to confront someone who's giving you the silent treatment [title] Consider what else is happening in their life. [step] It could be that they're not actually giving you the cold shoulder. Maybe they, or someone they care about, is ill or having personal problems.