In the log of a slave we see this:

JNLP agent connected from /10.1.29.208
<===[HUDSON REMOTING CAPACITY]===>Slave.jar version: ?

http://scrat:8080/hudson/computer/<computername>/log

This computer is connected via jnlp by launching the slave like this:
javaws -uninstall
javaws http://scrat:8080/hudson/computer/<computername>/slave-agent.jnlp

We start the slave in this way since we need UI interaction on the desktop by the processes started by Hudson slave.

We have a groovy script to check all versions of the slave.jar to see if they are up to date.
All slave started by the Hudson master are updated automatically.
Slave started manually do not update the slave.jar

— groovy script —
nodes = hudson.model.Hudson.instance.nodes
for(node in nodes){
if(node.toComputer().isLaunchSupported())
{
print "AUTO "
}
else
{
print "MAN "
}
print node.getDisplayName() + " "
def matcher = node.toComputer().getLog() =~ /.Slave\.jar version:./
matcher.each

println ";"
}