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The guidline you want to look out for is cloaking, showing one thing to the user and one to the serch engine. If you are hiding the text and not showing it to the user then you have a problem. if you have some way that the user can click and then see the hidden text you should be ok. how does google know the difference? i dont know if they can algorthmicaly, but if you rank well a compeditor will look into your site and report you if he can.