The low tech way:
- Measuring tape Simply take it with you and measure five bikes that are comfy, and five that are not.
- Digital camera Ask someone take a picture of you sitting on the bike, then review the picture to analyse yourself the sitting positions, both those that feel comfortable, and those that feel uncomfortable.
- Chainstay. You need long enough chainstay for panniers. My recommendation is 45 cm. Below 42 cm your feet can hit the panniers even with small feet.
- Comfort. You can select the ideal frame size based on different criteria: aerodynamics, efficiency, comfort, resulting speed, etc. For touring, the comfort is 10x more important then speed or anything else. This is because you sit 3-6h a day, for many days. On a tour, you get further by pedaling more and less by pedaling faster. Example: you can be 30-50% faster in an uncomfortable, but speedy position, but you can maintain that only for 1-2h. However, in a comfortable position, you can stay even 6-7h in the saddle.
I'd rather select a slightly smaller bike vs. a slightly bigger bike.
By buying a second hand bike, what you risk is hidden failures vs. the gain of lower cost.
- Steel frames: check for "bents" and corrosion
- Alu frames: check for cracks.
Bents usually come from accidents (falling with the bike), and cannot be corrected. You feel it only in the first moments of the riding, after a couple of minutes the feeling can go away. This cannot be repaired. I do have a steel bike that is bent.
Cracks come from big impacts and overload. This can be corrected by welding the alu frame, you need to find an alu-welder. I do have an alu bike with a crack (high quality brand, I didn't take yet the time to find a welder and get it repaired.
For more details, see my response here:
How do you distinguish a touring bike...