Forward is in the +X direction in Unreal coordinate system terms.
You can find out how the engine defines “forward” simply by looking at the ForwardVector constant on FVector class (there may be a way to inspect this through blueprints, but I haven’t tried).
FVector::ForwardVector = FVector(1,0,0); // X is forward
Similarly, you can look at the UpVector constant to get a feel for “up”:
FVector::UpVector = FVector(0,0,1); // Z is up
In terms of importing models, I’m not sure how the pipeline handles models made in a different coordinate system, but I would imagine the Engine would somewhat respect/convert it to the right rotation based on the file. But if you are building out your blueprint or construction, you should definitely point the ‘front’ in the +X direction.