IX vendor lists are pretty much public information. There's plenty of ways to find the information but you have to work backwards from how you're thinking. The way you've phrased this is to find which IXes an ISP is connected to, but you can only really find what ISPs are at which IXes.

For example, TorIX, a limited list is literally on their main page of their official site: https://www.torix.ca/

Rinse and repeat for every IX.

If you can find which business segment for an ISP handles consumer connections, you might be able to find them on the peering db ( https://www.peeringdb.com ), which, if you dig hard enough, you could get a list of which IXes the ISP is linked to (aka, peered with). Taking a major ISP here as an example, Rogers, there are eight networks with "Rogers" in the name. A couple are probably not the Rogers ISP/Telco company, but it looks like six might be, each is some business segment. Now, I know that Rogers does most of their internet services using AS 812, so I'd start there, and I can see they're linked to Toronto, Montreal, Vancouver, new York, Chicago, Seattle, and Ashburn. The question I can't answer is, does Rogers have a non IX-IX link (something geographically divergent) from, say, Toronto, and their clients there, over to Montreal/new York/Chicago/(etc). Some kind of interconnect that doesn't require the IX to exist? If I'm in, say, London, Ontario, about two hours west of Toronto, (where my closest IX would be Toronto), and the IX in Toronto goes away, will there be a way for Rogers to get my traffic out to the internet? Or does my traffic go to Toronto and get relayed from there every time?

There's no possible way to know without having intimate knowledge of the interconnects that Rogers owns and where they are physically run. Even if that fiber link looks like it goes straight from the London area to, say, Chicago, how do you know that the physical fiber doesn't go towards Toronto, land in the IX then get switched across an IX-IX link, to the Chicago IX? It's impossible to know.

Turning it around on Montreal, if I'm located in say, Ottawa, which is pretty much midway between Montreal and Toronto (closer to Montreal), and the Montreal IX ceases to exist, will my traffic have a viable path to the Toronto IX? In the same way, I have no idea. It could bounce through Montreal's IX before going to Toronto over an IX-IX link.

I suspect that the Ottawa situation is much more likely to have dedicated fiber running from there to both Toronto and to Montreal, since it's the capital of Canada, and having that kind of geographically diverse path to a couple IXes, would be a requirement for any government contracts, so it's likely in that case. But London? Not so much.

You can't just trace route this stuff either, since the ISP will have a preferred path which saves them money. That preferred path is likely going to go through the nearest IX regardless if your traffic should head the opposite direction. You can maybe dig through BGP via a looking glass (such as hurricane electrics looking glass at https://lg.he.net ), but that only tells you if the IP range is advertised as accessible from a location, not how it gets from that location to wherever it's going (IX-IX links only, or is there a geographically diverse path?). It's an ok indicator, but definitely not a definitive answer. Another good indicator is whether a trace route shows IX related links or not, but without knowledge of whether those links that don't indicate transit through an IX are actually in an IX/DC connected to an IX or not. It also doesn't tell you if those are the only links for that path, as the ISP has no responsibility to expose their internal routing information to the public. The IX-IX paths may just be preferred, and the direct paths are simply on standby for them if there's ever a problem with the inter IX path.

Long story short (and TL;DR): you can get some decent hints but unless you work for the ISP, you can never really know. The non-IX paths don't necessitate that they are ever used in the internal routing by the ISP, whether they exist or not, and nobody will just go around spilling those beans whenever the question is raised.