hmmm.... form a networking standpoint... if you need to know this information then you've got the ability, and if you need to ask... you might not need to be doing what it takes...
that said... no there is not a dos command... if you know the ip address of the person you can ping them to see if the computer they use is on the network...
if you know the hostname of the computer they work from you may try to map to thier c drive \\"hostname"\c$ should work...
if you want to know if they are logged on, you can try from a cmd prompt netsend if it's not disabled on the network,
send them and IM using AIM... or another messenger
or just call them...