Looks more like a Light Sussex than a Delaware with the pink legs and solid black through the tail (no barring). At this point, it's the comb that points to male. Pullets don't start pinking up in the comb until they are just a few weeks from laying eggs. That usually happens sometime after 4 or 5 months old for most breeds. At about 10 weeks old, male specific feathering is just beginning to develop. It can take several weeks for the male saddle and hackle feathers to be clearly visible. They are the long, thin, shiny feathers that only males have.
You know what, I missed that. With the leg color, that is a Sussex. Just as well, they're both beautiful breeds.
As for your question, Farmgirl, there are a few different techniques to sexing, and a couple of characteristics to look out for. These include stance, feet, voice, comb/wattle, personality, and feathers.
Cockerels will often stand tall, stretched up, and hold their tails in the air, rather than the horizontal stance of pullets.
Cockerels often have significantly larger feet than pullets.
Cockerels will have a raspier, deeper voice than pullets.
Cockerels will have a large, red comb and wattle, and it will grow faster and darker than pullets.
Cockerels will often be more bold and unafraid than pullets.
Cockerels will, starting around 12-14 weeks, begin to grow sharp, long saddle feathers, very distinctive from those of pullets.