Hi,
Just in case you dont get any other more educated responses...
You can order a small LED with a certain mcd range and lite it up with some current and take it to the place under the sun you want to be able to see the LEDs at. If it appears bright enough, you might go with that. If not, increase current and see if that helps, or try another LED. Sometimes nothing beats a good ol' fashioned test or two to make sure we are getting what we really want out of a circuit (or LED or other device).
Sorry i cant offer a more concrete idea, but i can tell you i had this problem myself and it isnt that easy to solve because sometimes the sun is so bright you really have to have a shade hood of some type. I ran into this problem when i replaced my "oil" light on my automobile with an LED. It was a high brightness LED, and driven at a healthy current level. Still, i have trouble seeing it if the car is parked in such a position as to allow the sunlight to directly penetrate the area where the LED is now located. It's plenty bright when the sun doesnt hit it directly, but other wise it's a bust.
In other words, sometimes you need a hood as well as a high brightness LED to be able to see the LED as well as you really would like to see it.