Back when lots of devices came with a thumb control at the bottom of the screen I used to advice that elements with 'hover' states should also have 'focus' states that perform exactly the same actions - this way, as a user scrolled through the screen with their thumb control, the hover states would be activated for each element they passed. This allowed us to alert the user to the presence of clickable buttons, links and tooltips.
Now, however, these controls are less common so we have to think more carefully about the way we design touch interfaces.
There are a few potential solutions including:
- Surface the hints/tooltips as visible text on the page - Don't hide the text in a tooltip but display in in the page as default alongside or under the field name.
- Add a 'hint' link - Add an icon (usually a question mark '?') near the field name so that a user can choose to look at the hint/tooltip by tapping the icon
- Contextual tap - add a long touch (tap and hold down) function that brings up any contextual controls associated with the relevant element that may include the hint/tooltip
As with most things in UX, I would advise testing with real users to determine the correct approach for this product.