just query objects and get first item out of the result (the same like in relational database)
to get it by Guid ID:
using (IObjectContainer session = this.GetNewSession())
{
Dummy result = (from Dummy item in session
where item.Id == Guid.Parse("....")
select item).FirstOrDefault()
}
the result will be either null if item doesn't exist or the object found
other option is to get it directly by internal ID such as (or even UUID):
long id = ....;
using (IObjectContainer session = this.GetNewSession())
{
Dummy result = (Dummy)session.Ext().GetByID(id);
}