You can find out by taking Riolu to a Happiness Checker of which in Blue's House in Pokemon FireRed and LeafGreen, you can find one in Goldenrod City in HeartGold/SoulSilver, you can find checkers in Verdanturf Town and Pacifidlog Town, you can see the checkers in the Fan Club in Hearthome City and in the Eterna City Pokemon Center, in Black and White you can find them in the Fan Club in Icirrus City and in the building eat of the Nacrene City Pokemon Center and in X and Y you can fnid them in the house west of the Santalune City Pokemon Center and in the Laverre City Fan Club additionally the sayings that you want to hear from them would include things such as it being extremely friendly toward you, it couldn't possibly love you more, it's amazingly friendly towards you, what great friends you are, it's super friendly to you, it must love you a lot, it simply adores you and it couldn't possibly love you any more than it does now.
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You can go to Veilstone City and take your Chansey for a massage there. When its happiness is full, it will evolve by its ownself. Hope I helped.
Level it up when it its happiness is full
I think with full happiness (then level it up one level)It needs to have max Happiness AND you need to have the National Dex.
Go to the Pokemon cente in eterna and talk to the lady with brown hair and she will give you the happiness app (the one with all your Pokemon floating around and bumping into each other). Hold your stylus on apokemon of your choice and if 2 big hearts come out of it, its happiness is near if not full.
when you talk to it if it has six hearts around it that means its at max happiness