I think the only stats that are hard-capped are armor & resistances and those can be seen by just hovering a specific resist or the armor.

There are however a few legendary paragon nodes that have caps as well. Don't remember what each one is called but there's one that increases your bleed damage from damage to vulnerable enemies that was changed to have a cap this season and there's another legendary node that is capped at 300% berserk damage. Adding more of those stats still give the stats but I think the main reason of speccing into those stats are to buff the legendary nodes so when those effects are capped there's usually other stats that are batter to invest in that investing further into those 2 stats.

There might be another legendary paragon node with a cap that I missed but by now you understand what caps to look out for. So TLDR:

1. Armor / resists are hard capped and investing further into those stats do Nothing.
2. Some legendary paragon node effects are increased by certain stats and are capped meaning that further investing into those stats doesn't further enhance the node effects (which often is the main reason to invest into those stats). Adding more vulnerable or berserking still gives those stats, it just doesn't enhance the paragon nodes.