> "Can we actually see them?"
From a folklore perspective, yes. It's called the second sight in European folklore, allowing one to see one or more fairies.
Also fairies can and do make themselves visible, often as an incorporeal form.
> "If we can't, then how do we feel their presence?"
There are other ways of feeling than visuals. Scents, sounds, feelings, hedge-crossing and other out of body expirences, even divination.
> "What do they prefer as an offering?"
Depends on the type of fairy it is. It is important to remember that the term fairy is synonymous with spirit. They are not a unified class of beings, but rather spirits. They can be spirits of the dead or spirits that have never had flesh. They can be simple wights of a plant, the spirit of a region, personifications of seasons, gods even.
Typically though, milk and honey, or bread often is left for them.