A scrollbar is the classic affordability cue that cries "scroll me!" though they can be ugly/space wasting (but not unheard of) on mobile touch devices. On a tablet though a 7px scrollbar really doesn't eat much space and can have the helpful benefit of showing how much content exists; many Android apps will show a "hidden" scrollbar that only appears when you scroll to help show your position in a scrollable element.
If it's a technical reason you're not using scrollbars, some js and other tricks can allow their use, such as iscroll JS.
If scrollbars are unwanted for non-technical reasons then you have to suggest the scollability. This can be done inelegantly by directly telling a user that X is scrollable on their first visit, but subtle cues are a more appealing and practical method. Try to make it clear that there's something beneath the fold; if text extends beyond the fold or other element is partially obscured users will quickly get the hint (balance this with aesthetic appeal, don't intentionally hide important things by the fold).
An interesting example of this suggestion is Cultural Soliton's website, as @RogerAttril put it in chat,
The scrollbar is not the only cue on that page :- the circles on the background go below the fold - like something bubbling up from below