The used totes I see typically have a WHMIS/HAZMAT label still attached. That lets you track down the material stored in it, and determine its risk. Polyethylene is quite porous, so beware if it held nasty stuff.
I assume you are meaning used IBC totes from a reseller. My experience is that most sellers will tell you what was in them before, usually "food grade" is a selling point and demands higher prices but that is what I would want for water storage. Obviously buyer beware, if they say it had peanut butter in it but it smells like harsh chemicals definitely pass. I know some seller promise that the totes have been "cleaned" of what was in there before but if it wasn't food products I am unsure I would personally trust it.
John Y. makes a good point: sellers of food grade totes receive a premium price, so they are very up front. I recently received three liners that contained apple juice destined for a cider manufacturer. You could smell it. No problem there.
Don't get one unless it has the label still on it from the original product it was filled with. Some things would be obviously safe, like a food product. Some obviously bad, like anything with a hazard label. And for those unknown ones, you can easily look up the SDS (Safety data sheet) online. These are required to be available for every trade name a product has. The SDS will show you every detail of potential hazard.
Do your own due diligence!! (homework) I have gotten some IBCs from local manufacturers that have held various things:
vegetable oils used for plastics molding (probably safe)
agave syrup (natural sweetener, definitely safe)
sodium lauryl/laureth sulfate (not so great, harmful to aquatic life) seller represented as "a derivative of coconut oil" since that's the odor one smells from it (and technically it is). Not very useful to me as any part of building an aquaponics system... It might rinse away with LOTS of water/pressure washing/soap, but the investment in time is better spent finding suitable IBCs in the first place. I'll use them for thermal mass in a greenhouse.
Here's what https://en.wikipedia.org/wiki/Sodium_laureth_sulfate says about it.
Use your good judgement. If buying directly from a user of the "product" and it is a food business, it's likely a known quantity. If buying from a reseller, check and be sure, identify the actual contents... also make sure you are given what you are paying for (the right ones, not the "right here" ones). If someone isn't forthright, or they aren't sure, I'd pass on it, You don't need to inherit a disposal problem.
I know I'm the one being a bummer here, but has anyone considered the plastic content of the IBC totes and their effect on the food you're going to be consuming?
I don't want to sound like an alarmist, but there's increasing evidence that microplastic pollution may be causing fertility levels to drop cross-species. I will try to find a link to one of the many articles citing this evidence. It backs what Paul has been saying for quite a while now, anyways, regarding plastic being a bad choice.
Sorry to shit in the soup, but I feel like it's the used tire conversation again, except worse, because the totes perform a useful function that it's not really easy to replicate with anything unless you're a cooper. And, you can't encapsulate them and have them work for you. Unless you coated them in clay... Adobeed IBC totes? Mix in crushed biochar and you might end up with an effective, stable barrier to toxic leaching...
A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects.
-Robert A. Heinlein
I consider them to have usefulness in aquaponics, where after a few years of active biological life processes in the container I don't feel they'll be leaching anything in biologically significant quantities. But I can't prove that.
I got one tote really cheap (as part of a bigger buy I did with a guy who had a trailer for hauling them) that came with a label indicating prior use to contain a product that was, basically, dish soap, although I think it was used at a car wash. I wouldn't store drinking water in it but I think it will make a fine goldfish pond in which I might eventually grow submerged aquatic plants.