Random question, but you lot seem to know your stuff...

Is there an easy way to identify if a hybrid bike is using road gearing or MTB gearing? Specifically I'd need to know if I could put a road crankset on it (at least without having to bodge anything). I'm putting together a new trainer bike, and a hybrid seems like it'll be a closer match to the MTB geometry I'm used to, but I need access to the bigger gears that you can get on road bikes (53 tooth chainring).

Cheers.

The width of the bottom bracket shell should be a good indicator. As a general rule a road bike will have a 68mm shell, and an MTB 73mm.

In the case of an external bearing bottom bracket, you can still run MTB cranks on a road frame by spacing the bottom bracket, but not the other way round i.e. road cranks on an MTB frame.

Do you have any of the parts already or are you starting from scratch?

Not right now, but the plan is to throw an Ultegra R8000 Crankset on it with a view to eventually getting it kitted out with Stages L/R power. The other option is buying a smart trainer... but if I do that there's a bunch of other expenses to deal with (like finding a way to flatten out my garage floor).

Oh, and my current trainer bike has a 68mm BB (despite being a MTB), but if I put a road crankset on it, the big ring would hit the chainstays.

Not sure if i’m misunderstanding something here. Surely you can identify if the bike is equipped with road or mtb gearing by a) the name of the group set and b) the size of the gearing.
BB size as far as i’m aware is not set. I have had mtb’s with both 68mm and 73mm BB, the same for road bikes too. Not to mention all the weird and wonderful BB ‘standards’ knocking around at the moment.
Also, do you really need a greater than 50t chainring? Coupled with the correct cassette a compact 50/34 chain set would be more than adequate for most people’s use

It's for low cadence stuff on the trainer

As for identification... many of these hybrid bikes seem to come with extremely strange mixes of equipment. I've seen a few with road up front and MTB at the back. Plus of course many of them come with no-name crap... which I don't really care about, because I'm going to replace it all, but I still need to work out if the frame is going to work out before I buy it.

I'd wager anything with 50t or above would be safe, but it's not exactly common to see that on your average hybrid.

Guess that makes sense. Just get the biggest gearing you can then but as you say an off the peg hybrid is unlikely to provide what you want.

Do you want to ride this bike only on a stationary trainer? Or do you want to be able to take it out for rides too?

If the former, geometry shouldn't matter all that much, as you should be able to get all the contact points in the right places (e.g. with comically long/short stems or seatposts).

If the latter, then things might get a bit more complicated to satisfy your gearing and geometry needs, but there will probably be a way. It'll just be hard to come up with without knowing exactly what you're working with.

One thing I'd suggest looking for a square taper BB and crank set-up, as you'll be able to play around with chainlike more. This could help with chainring/chainstay clearance.

Shouldn't matter too much, no, but obviously it'll be somewhat easier if it's at least in the ballpark!

(and no, it'll only ever live on the trainer)

Forget the hybrid and buy a gravel bike, most of those have got inappropriately high gearing straight outta the shop