Generator Transfer switch to subpanel instead of main house panel?

Installing a manual transfer switch for our generator to run some lights/outlets in our house. Nothing major.

We have a main panel on the side of our house that then feeds to a panel on our garage panel. The garage panel has all the breakers we'd like to wire into a transfer switch.

All of the research I have done is wiring the transfer switch into the main panel. I'd like to do it to the subpanel, but am unsure if the grounding the neutrals are okay at that location. The entire electrical system was upgrade a few years by a professional. I read some about the possibility the subpanel does not have a ground going back to the main panel. Is this a possibility? Can I tell by looking inside the subpanel?

Any help would be appreciated - I was fully prepared to move forward with this until I realized the panel I was going to install into is just a sub-panel

Thanks!

If you don't know or haven't looked as part of your plan and research, then you really NEED to at least consult an Electrician for a paid quick minute or 2 stop-by and walk-through. An upgrade a few years ago should mean and guarantee (being a short run in a single structure) that you have a bonded ground to the Main and an unbound neutral in the Subpanel and can proceed as planned.

- However, if you're saying the Subpanel would just need to be energized and ALL of the circuits in there are what you'd want. Then, consider or ask the Electrician if you can much more simply use a Single Load Manual Transfer Switch to JUST wire the Subpanel's Breaker in the Main Panel. A LOT easier, cheaper and real quick to take with you when you move out of the place...if desired.