If you have ANY kind of elastic around yr body, and yr bodycon dress ain't betrayin' it, then... yr bodycon dress ain't bodycon enough. End of.
If you MUST have something between you and the dress -- say it's from Rent the Runway and you just ain't takin chances like that -- then, wear a slip. Yep, a slip. Like yr grandma would, except today's slips are actually made of stuff that doesn't destroy yr silhouette. You can even find slips made of Spanx-type stuff that will help you rock that rockin' dress just a little bit more.
PS -- These are the kinds of dresses that make ALL them #girlproblems SO totally worth it. Nyah nyah, boys, with yr lack of fashion options. ::thumb in nose, waves fingers::
My only bodycon dress has a textural pattern to it, so it helps to hide pantylines. I would suggest going without underwear if you're worried about it being too visible (and if you're comfortable with that).