Okay, thanks to everyone for the advice. What I'm really after I
think is that it's flat. I don't actually want it level, since it
currently slopes a little down toward the sump pump, which I think is
good.

The concrete is 10 years old, and even with the super heavy rain
storms this past year, we have NO water in the basement. The original
owners installed a "Tuff-N-Dri" waterproofing system, including a
"Warm-N-Dri insulating foam on the outside of the poured concrete
walls, so even they aren't really as cold in the winter as some
basement walls I've felt. So I'm fairly confident that the basement
will remain dry, unless our water heater blows up.

So, basically I want to put down engineered hardwood flooring (DIY, I
hope). My plan is to put down the Delta-FL underlayment, then the
moisture/sound barrier, then the flooring. I figure the Delta-FL will
do double-duty, one as insurance against some water eventually coming
in (or from the water heater in case of catastrophe), and it should
help make the end floor not so "hard". Several people have noted
that the Delta-FL makes the floor "softer", more like a framed
floor.

Anyway, in reading forums on this, several people have mentioned
needing to level the floor with leveling compound, but all those posts
seem to indicate a visibly uneven floor, whereas mine looks pretty
good to the naked eye, even putting my face to the floor and
scanning. But I'll take the advice here and try a bowling ball,
marble, and water test. If anyone has any other advice based on this
new info I've provided (on the even-ness of the floor or otherwise),
it is all very much appreciated.

Thanks,
M