How to search for observation with any field set

Is it possible to search for observations that have any field set? In other words, with one or more fields set, not caring about what particular fields they may be.

as far as i know, this functionality does not exist. using the API, it would be possible to do this kind of filtering on the client side though, although it would only be a viable option in cases where set of observations that you would search through yourself is not huge.