I think you might be conflating system bus with the CPU's register size. CPU's have independent buses these days to access the various components of the system, bus size not-withstanding. In other words, a CPU could be '64-bit' and potentially have a 32-bit bus on the PCI channel, etc. The 64 vs. 32-bit issue is in regards to the register size of the CPU; that is, how much memory (among a few other things) the CPU can access.
If you're referring to a specific bus (like the front-side memory bus, etc.), then you'll have to find out what the specific processor is in the machine you're looking at (e.g. Intel Core i7 model XYZ, etc.). But if you're purchasing a laptop, unless you're looking at a high-end gaming laptop, then usually those kinds of metrics are locked to whatever the motherboard can handle. So you might have a high-end CPU that can handle DDR4 3000, but if the motherboard only supports DDR4 2133 RAM, then that's what speed you'll get.
To answer your questions more directly:
How can i tell whether a laptop is 32-bit or 64-bit?
If it's a modern laptop (i.e. anything made within the last 10 years), then there's a 99.99% chance it's a 64-bit CPU.
To verify, you'll need to get the CPU being used in the laptop then do a quick search on the CPU manufactures website (most likely Intel or AMD).
Are there still 32-bit laptops out there in the market?
Yes, but most are on the low-end/embedded market, and if you're purchasing a new laptop with Windows on it, again, there's a 99.99% chance it's a 64-bit CPU.
Can a 64-bit processor run on a 32-bit bus architecture?
Again, you're conflating bus with register size. Any CPU must have a compatible main board to connect to, the main board can have various buses of various speeds and bit-sizes (e.g. PCI, PCI-X, USB, SATA, etc.), but you cannot run a CPU on an incompatible board. As an example, I cannot stick a 64-bit AMD into an Intel based motherboard (regardless that they're both 64-bit), just as I couldn't stick a 64-bit CPU into a board that was designed for a 32-bit CPU (and vice versa).
That being said, most 64-bit CPU's (and OSes like Windows) have a layer built in that lets you run 32-bit software.
Again, if you're purchasing a new laptop, unless you're a gamer, the biggest things you'll need to look at are CPU speed and amount of RAM that comes default on the system and make sure that will serve your needs.
Hope that can help.