Well, there are many indications. First, the scene looked as if the room had been prepared for a romantic honeymoon. In true Southern Gothic style, the place was also run down, and the flowers and all the things that once may have been beautifully set up and once fresh and colorful had seen time, dust, and abandonment go by. Hence, the scene was a morbid one where we can picture a very impressive scenario with dead flowers, dusty curtains, abandoned draperies and linen, the smell of dampness and humidity, and a huge mess. That, in itself would have been terrifying and disturbing enough.
Yet, as the story goes, on the bed there was the indentation of a head that had been sleeping there, and from it came that iron-gray here. Yet, they had made the connection there and then that Emily had been sleeping with the corpse of Homer Barron because, next to that pillowed, under the nightshirt she had been seen shopping for him, and next to the crystal and silver man toilet set, there was the rotten remains of Homer in the room.
Hence, as you can imagine, the idea of necrophilia cannot escape one much.