The answer is... "It Depends".
So the next obvious question is "Why do you want to know?" Because that might get you a better answer than what follows... Can you elaborate?
There is data in the internal logs regarding the contact between the indexer and forwarder, but those logs roll and usually... that final rollout is to deletion. So it depends.
If this is a new-ish (the _internal index is set to roll every 30 days) then you can take a look here:
index=_internal source=*metrics.log* "fwdtype=*"
or if you are using a DS here:
index=_internal source=*metrics.log* phonehome
These give you the data available to you (although not the search, there are countless resources for searching those bits...).
the UF also has internal logs that most folks are forwarding to their indexer (but you fall under the same 30 day roll).
If you feel it's been phoning home since it was installed and you just want to know when... if it's never been upgraded you can check creation data of default config files in $SPLUNK_HOME/etc/system/default as those are dated at the last upgrade (or the initial install). mind you, this assumes you started it immediately, and that there isn't some overlying distribution system messing with the dates.
With Splunk... the answer is always "YES!". It just might require more regex than you're prepared for!
