I can't speak for Spanair as I don't fly there and have no insight into their operations. In general, two systems are used for determining weight of passengers and cargo. One involves a standard, average weight, or assumed weight, for passengers (eg, average adult female weighs XXX, etc), and these numbers can vary somewhat with the operator, and are subject to change. Baggage may also be assigned a standard weight unless the bag is determined to be over that by observation or actual weighing, when it may be classified as a "heavy bag" and given a higher standard value, or an actual weight.
Where weight appears to be different than standard, the operator may have a policy requiring an actual weighing or in the case of a passenger, asking the weight...though that's seldom done for obvious reasons.
The other method involves actual weighing of items to go on the aircraft. While that's done to a point, it's seldom done with passengers, and not always done with baggage.
The crew is always furnished with weight and balance data prior to a flight.