are you 100% sure theres no sticker? maybe its on the psu's other side and you need to open the other side of the case to see it ,
what about searching up the recipe,or looking online at the pc specs if you bought it in one peice , you might find the info printed on a circuit board or some part inside the psu however openning up the psu will void your warranty but if you gonna do it then first unplug from the wall socket also disconnect it from the motherboard and other devices and do not attempt to touch the various componets inside the psu as some of em might still hold electricity so be carefull , anyway if you ask me if it got no sticker its a plain bad sign , whats your full system specs? if youll give em out ill be able to tell you what psu you need