How to find start/end of ramp in revit, perhaps with sketches?

Assuming that your Ramp is a FamilyInstance :

var fecRamps = new FilteredElementCollector(doc)
    .OfClass(typeof(FamilyInstance))
    .Where(pElt =>
    {
        int lCatId = pElt.Category.Id.IntegerValue;
        return lCatId == (int)BuiltInCategory.OST_Ramps;
    })
    .OfType<FamilyInstance>()
    .ToList();

List<XYZ> lRampLocs = new List<XYZ>();
foreach (var pFam in fecRamps)
{
    var fLoc = pFam.Location as LocationCurve;
    var fRampSide1 = new XYZ(fLoc.Curve.GetEndPoint(0);
    var fRampSide2 = new XYZ(fLoc.Curve.GetEndPoint(1);

    lRampLocs.Add(fRampSide1);
    lRampLocs.Add(fRampSide2);
}

Every FamilyInstance has a Location and you can cast the Location as a LocationCurve. From the curve, you can get the end points via the Autodesk.Revit.DB namespace.