How to locate floor joists

Not doable at all.

Old houses have 3/4'' thick wood planks as subfloor, not 8x4 sheets of plywood like today's houses, so I would say that it's ok to consider your hardwood floor the subfloor. That's if you're putting another wood floor on top.

However, for the purpose of tiling you need an additional layer of plywood to reduce deflection that will cause the grout to crack eventually. And on top of that you would install your cement board. The cement board itself provides no structural support and is only used as a decoupling between a wood subfloor and the tiles, so the lateral movement of the wood is not going to cause the tiles to pull up. You can always use a durock membrane to reduce thickness, but you still need a floor that will not deflect.

If you really want to do this right you would take out the old floor down to the joist and install some OSB and then do what you want...

You're already putting lots of holes in your ceiling, which will have to be patched. As long as you don't mind some holes, try this:

Use approximately a 1/4" drill bit to drill into a likely spot on your ceiling. If it hits wood all the way through, you've just found a joist. If it goes through the lath and hits a void, get a piece of wire or an old coat hanger. Put two bends in it so it looks like three sides of a rectangle. Make sure that the top and bottom sides are the same length. Work the top side into the hole. Make it "turn the corner" in the hole so that the top piece of the rectangle ends up parallel to your floor. Now spin the wire until that top section of the rectangle hits something and stops. If you made the bottom section the same length as the top, then the bottom section will be pointing right at the location of the joist.