How to check static mesh has valid mesh property?

I have created a UStaticMesh variable in c++ with UPROPERTY, and in the constructor I can create new object for this UStaticMesh with NewObject or CreateDefaultSubobject, and the question is at the first place there is no mesh set for it, in the Details panel I can set a new mesh for it, how can I check if it’s already set or not in BeginPlay?