There are ways of knowing. ;-) Technically, a block mostly just prevents an account from replying to you again — which is the primary goal when someone writes

There are ways of knowing. ;-) Technically, a block mostly just prevents an account from replying to you again — which is the primary goal when someone writes a nasty or annoying reply — but there are various ways of seeing another account's posts, such as private windows. And if it's not a mutual block, it may be even easier.