Wiki

public static bool GetFieldByID(this SPList list, Guid fieldID, out SPField field)
{
    foreach (SPField spField in list.Fields)
    {
        if (spField.Id == fieldID)
        {
            field = spField as SPField;
            return true;
        }
    }
    field = null;
    return false;
}

Usage:

Guid fieldID = new Guid("...");
SPField field;
if (list.Fields.GetFieldByID(fieldID, out field))
{
    // do something with the field
}