How could I find out if there are any backups going on?

Couple of options (as this is nfs and not ddboost).  You can run net show stats (which is same as netstat) and see which sessions are established.  This alone doesn't mean always data transfer of course, but you can also use net iperf to measure traffic on your DD and see if anything is coming in and from where (you can use nfs show active to see active nfs clients).

please see the attached.

I don't know much about "net iperf client IP", but as attached, commands on both different IPs all  show me connection failed. Why? these two IPs came from net show stats, and as ESTABLISHED, so should not be failed, right?

Please help me to interpret the outputs.

"nfs show active" shows you mounted clients, but they are not necessary writing any thing to it, right?

Ah sorry, not ipef, but rather tcpdump (tcpdump captures network packages). 

Hi, please find the below. there are alwasy some packets captured on any ESTABLISHED clients. only those with large number of packets could indicates backups going on? What about "nfs show stats", could it tell you if backups running?

# net tcpdump capture x.x.215.18

tcpdump: listening on eth0b, link-type EN10MB (Ethernet), capture size 96 bytes

51 packets captured

51 packets received by filter

0 packets dropped by kernel

Doesn't mean much to me since no one knows what is inside (time duration, package size, etc).  Wouldn't be easier for you to check some client and simply see if backup is running or not?  Obviously what drives backup should have some kind of monitoring and that monitoring should be your source of information.