Gotta love all the generalities people throw out when discussing this topic. Yes, there are some simple rules an EXPERIENCED person who has lots of structural construction experience can play with to determine if HIS particular situation warrants a call to engineer or not. But unless that person is willing to bear the moral responsibility of ruining someone else's house if he's wrong, suggesting an engineer is the right way to go.
I have a good working relationship with an engineer that ran the numbers for my place and my 210. He didn't charge me a dime, but Wardog is right, hiring one off the street will not be a cheap proposition. Thus why so many skip this part of a large build.
I don't blame an engineer who wants ceiling drywall torn off to look and see just what mechanical mess is run through floor joists. I'm an HVAC journeyman and I've seen all sorts of moron's cutting TJI's in the wrong spots to run ductwork. Yes, you are allowed to drill through TJI joists following the manufacturer's charts and recommendations, but not everyone follows that and believe it or not, building inspectors don't catch everything. If I were responsible for someone else's house, I'd want to see everything too.
Wardog - while I agree whole heartedly about many things you say, you also make a pretty strong point about why an engineer is the smart way to proceed. I have to disagree with an engineer being a waste of money in certain circumstances. If someone has no construction experience, and can't tell where bearing loads are being transferred and how they are doing so, engineers are a wise choice.
I am also a NFPA registered Fire Fighter and worked on a city Fire Dept full time for a number of years and I can tell you from first hand experience, buildings are not designed just to stand while you are going about your daily life and raising your family. They are also designed to stand under certain catastrophic circumstances. That floor must not only withstand the 4000-5000 lbs of the tank and related equipment and furniture etc, while you're sitting there watching TV, but it must do so during a minor earthquake, freak storms, and a potential fire below it.
Anyway, just a few things to consider. My comments a by far not all inclusive, just adding some points to ponder. Oh and a link I always post to questions regarding floor loads:
http://badmanstropicalfish.com/articles/article28.html
PPS, for what it's worth, in my experience, if the lower floor is visible, you should be able to find an engineer to tell you what's what for well under $2k.