If there are screws, they can be really difficult to see. One trick is to use a very thin metal spackle knife and insert it between the bolster and the side of the gunnel. If it goes in more than an inch or an inch and a half, there probably is no screw in that area. Move the knife along the entire bolster bottom edge and check. If there is a screw you will hit it. The screws if any will be where a grommet is on the bottom side of the bolster but there will be a lot more grommets than screws. If you can't get the knife between the fiberglass and the bolster at all, there may well be a screw in that area.
When reinstalling, plug any old holes with caulk or 4200 / 5200 before reinstalling the bolsters. It is just about impossible to reuse the old holes. Shoot a new screw into one of the grommets at a steep angle so that it hits the glass fairly perpendicularly. Two or three screws per bolster are plenty. One if fine for short bolsters.
The bolsters as mentioned are pretty tight without the screws but in some cases you might benefit from having them screwed in. (like when someone holds onto the bottom of the bolster while they lean overboard or when climbing onto the boat over a gunnel)
Another advantage for the screws is that it makes it a lot more difficult for someone to 'borrow' your shinny new bolsters.