(UE 4.18)
In the editor
If you look around the source code:
bool UDebugSkelMeshComponent::ShouldCPUSkin(){
return bCPUSkinning || bDrawBoneInfluences ||
bDrawNormals || bDrawTangents ||
bDrawBinormals || bDrawMorphTargetVerts;
}
(you could hardcode return false as an extreme measure)
In terms of UI in your content browser double click/open a skeletal mesh asset, then go to the Mesh editor

In “Show” → “Advanced” → enable “CPU Skinning” and voila!

You’ll notice that if you enable normals, tangents or binormals it will enable CPU skinning as well.
Now on the C++ side
In the code there is a function:
void USkinnedMeshComponent::GetCPUSkinnedVertices(TArray& OutVertices, int32 InLODIndex)
that will allow you to copy skeletal mesh vertices to CPU. Looking at the implementation you’ll deduce that to enable CPU skinning of a USkinnedMeshComponent you need to do:
USkinnedMeshComponent* ptr;
Ptr-> USkinnedMeshComponent::bCPUSkinning = true;
Ptr->UActorComponent::RecreateRenderState_Concurrent();
FlushRenderingCommands(); // from #include “RenderingThread.h”
Ref: [Get Access to SkeletalMesh Vertices in UE4 | Noah Zuo's Blog][3]
(UE 4.24)
That being said since 4.24 USkinnedMeshComponent::bCPUSkinning is deprecated and the more straightforward USkinnedMeshComponent::SetCPUSkinningEnabled() was introduced.
UI also changed, when I tested it in 4.26 it was still in the viewport menu but under the “Character” menu this time:

Bonus: to draw a bone’s skinning weight, go to “Character → Mesh → Selected Bone Weight”
As expected it will automatically enable CPU skinning too: