It is a very good question.
My nurse gave me advise how to check if Lantus is good for you.
3 hours after a meal (when your blood sugar is stable) check your BG and take carb-free snack.
2 hours after the snack check BG again.
If it went up you probably need more background insulin,
if it's the same as 2 hours before everything is ok,
if it went low you need to reduce your Lantus dose.
For me personally to take this test would mean not to eat much in 5 hours, which is impossible task.
I managed to do this once and my results showed my Lantus dose is good for me. But probably to do it only once is not enough.
I experimented a lot with my Lantus dose. I was told to take 16 units, I took 6, then 8, then 10, went back to 8, to 10 again,sticked to it for a while, then 12, 14, and now I take 15. But still I am not sure if it's right for me. My weight went up 11 kg in 2 months when I started taking 15 units, which is a good sign but maybe a little too much. I am thinking now to reduce it again. I would love to know how much I actually need..if only my pancreas could talk..
..but I think it will take years to get it right and a lot of experimenting.