For example, the page for Property:Population would specify [[has type::Number]]. "Has type" assumes its value is in the Type namespace, so you can omit the prefix Type:.
Since version 1.0, SMW allows many-valued properties. Types for these properties are specified by listing multiple types as the value of Has type, separated by semicolons. For example, on sandbox.semantic-mediawiki.org Property:Employment takes a link to a company, a job title, and a start and end date, so it specifies its type as [[has type:=Page; String; Date; Date]].