Healthy foliage is the best indicator of healthy soil, Sally said it well. Good color, lush growth, and sturdy stems are all good indicators that your soil has what it needs.
There are so many other factors, though, and foliage can change color for other reasons such as insect activity or fungal disease. So just be aware that plants in pots use up the goodies you put in there at a pretty high rate in hot weather, slower in cool. Be prepared to pot up, change soil, root prune and make other maintenance changes for your plants' health. Timing will vary depending on the plant.
If you are able to make some of your own compost, that is the Gold Standard of healthy soil for outdoor container plants. *there are articles and a forum about composting on the site here. Having a supply of your own compost on hand in a year or two when you need to refresh your pots will be the best thing you could do.