I have no experience in sampling fleas in houses. I know that there is a special trap equipped with flickering light, which is attractive to fleas and you can use it. You can try to use my method, which is a standard frame covered with white flannel cloth to which the fleas cling. Good luck.
In the old times we used a pan with hot water. You set it under bed and fleas will fall / jump in the water. A little bit of soap (detergent) in the water is needed to avoid fleas jumping out of the water.
When I moved into my North Florida house (1972), the previous owner had owned 3 hound dogs allowed free access to outdoors, thus mostly cat fleas had free access to the wall to wall shag carpet indoors. We vacuumed (powerful machines), trapped fleas with white tape, panels with Tanglefoot. and read about the light traps with flickering light and sticky surfaces. But nothing got rid of the infestation of fleas, new ones happily hatching and becoming very hungry. After a week of fruitless effort, I tented the entire carpeted room under polyethylene paint dropcloths over wooden chairs, weighting the outside edges with wooden sticks. Then full size "bug bombs" were sett off underneath and we left for 3 days. Upon returning home- they were all dead, and nymphs no longer hatched out. And No more inside/outside dogs or cats, ever!
For flea sampling in houses use myFleaTrap - see https://www.youtube.com/watch?v=GXanD-x4DpQ. This trap has also been used in sevral clinical evaluations of flea control products. This trap will catch the fleas after they have emerged from the cocoons. Be patient. Fleas can stay in the cocoons for quite some time if there is not activity in the house! I do not know if there is a Spanish distributor of the trap but will return to you regarding this.