How do you look up if someone had a will on the internet?

Wills are public information only if they have been filed for probate in a court. They are typically, but not always, filed for probate in the county of the decedent's residence. If that county has online records, you may be able to find out if an application to probate a Will has been filed there. It would be under the decedent's name. Typically, the document itself won't be available online. I've heard it's because we generally don't want sensitive asset information to be accessible online (for obvious reasons). If the Will has not been offered for probate, you won't be able to determine online whether or not a Will existed.