How can I check my Pokemon's happiness on Pokemon LeafGreen?

You can check it by feeding it something or teaching it a move. When the picture of the Pokemon taking the move or food, the happiness depends on how far it moves toward you, with three steps being the maximum.

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pitagoras:

you must go to pallet town to your rival house there it will be his sister now talk to her and she will tell you that your Pokemon is dirty and she will offer to groom one tell her yes and give her the Pokemon you whant to make happy that will make the Pokemon happier

now to know your Pokemon happines after all that i said you gotta put the Pokemon you whant to check the happines as thee 1st on your party now talk her again

now if she says this:

I don't like the way it glares at you. Could you try being a little nicer to it?

that means the Pokemon does not love you

now i am goin to put the thing she says and the happines points the your Pokemon has when she says it

Pokemon happines points:

I don't like the way it glares at you. Could you try being a little nicer to it? 0-49

It's not quite familiar with you yet 50-89

It's warming up to you. Trust must be growing between you. 100-149

It's quite friendly with you. Keep being good to it! 150-199

It looks very happy 200-249

It couldn't possibly love you any more than it does now. Your Pokemon is happy beyond Words. 250 +

when she says It couldn't possibly love you any more than it does now. Your Pokemon is happy beyond Words. this mean you Pokemon is ready now lvl it 1 lvl more and tah dah! it will evolve

hope i help you

peace out remenber pitagoras told you first!