First we can look into the appearance of the camera. Normally the dimension of camera with fix lens is less than camera with vari-focal lens. Meanwhile the vari-focal camera has auxiliary components, such as iris control, motor, gear.Secondly we can refer to specification of lens. Generally the focal length of fixed camera is a fix value, shown in the picture below:

While the focal length of the vari-focal camera is a range of value, shown in the picture below: