Note: this solution works only if the slaves have the same directory structure as the master. pwd() will return the workspace directory on the master due to JENKINS-33511.
I used to do it using pwd() functionality of pipeline plugin. So, if you need to get a workspace on slave, you may do smth like this:
node('label'){
//now you are on slave labeled with 'label'
def workspace = pwd()
//${workspace} will now contain an absolute path to job workspace on slave
}