That is a pretty vague question so the best we can do is give you similarly vague help. Start here:
Pardon me .What exactly I meant to ask is We have list of hosts not reporting in splunk .I am looking for best search to find out in UI why they are not reporting .
Start with the links above. If you get hung up, add a comment here.
I would start with this and see if they are even talking to the indexers at all.
index=_internal source=*splunkd.log host=(YOURHOST)
If this returns no results, there is no transmission. In that case, check the that the service is started, that the port is open. Then check the $splunkhome$/var/log/splunk/splunkd.log for clues.
If there is communication, chances are you don't have any apps in place in $splunkhome$/etc/apps.
Splunk is running, but hasn't been told what to do.
Try this search on your DS,
|inputlookup dmcforwarderassets | search status="missing" | fields hostname os arch forwardertype version lastconnected status | rename hostname as Instance | eval now=now() | eval DurationNotConnected=now-lastconnected | where DurationNotConnected<=2592000 | fields - lastconnected now | sort DurationNotConnected | eval DurationNotConnectedDays = round(DurationNot_Connected/86400,0)