Here's a query that I use to see all machines who haven't rebooted in 7 days. You can change the 7 to however many days you want.
select SMS_R_SYSTEM.ResourceID,SMS_R_SYSTEM.ResourceType,SMS_R_SYSTEM.Name,SMS_R_SYSTEM.SMSUniqueIdentifier,SMS_R_SYSTEM.ResourceDomainORWorkgroup,SMS_R_SYSTEM.Client from SMS_R_System inner join SMS_G_System_OPERATING_SYSTEM on SMS_G_System_OPERATING_SYSTEM.ResourceID = SMS_R_System.ResourceId where DATEDIFF(DD, SMS_G_System_OPERATING_SYSTEM.LastBootUpTime, GETDATE()) > 7