Applying QoS to a 10Gig Trunk port

So lets pretend I'm no whiz at QoS...

I currently have a situation where in one direction, Citrix traffic is getting tagged correctly (From the local router/switch) and going over QoS as intended towards the datacentre, though in the other direction (Datacentre to site) it is going as Best Effort (BE).

If we ignore the working portion, I need to workout where to apply the QoS policy to tag the traffic going back so its no longer BE. The direction of hops/travel shows as:

VLAN on VMWare environment -> 10Gig Trunk ports to Core Switch (VLAN on core switch) -> WAN Routers -> Other sites WAN Routers -> Other sites switches

Now, I have applied the tagging and policy on the WAN Routers at this end though it doesn't work (QoS is already applied on the other end), I know I need to tag closer to the source so my question is, do I/can I apply the tagging/policy on the Core Switch 10gig trunk ports or the VLAN interface itself?

I hope this is enough info, if not I can provide whats needed. Thanks in advance!