Some regiones can only be entered by performing a ritual, or on specific days, and merely seeing the boundary with a InVi spell is not enough to enter it. How would magi investigate how to enter a regio that is only open on a specific date, such as a solstice?
Presumably you would see the entrance even when it is closed- InVi does indicate it will tell you how to enter, it doesn't say how to enter immediately.
I don't think there's a general answer, but depending on the type of entrance/Realm I would generally suggest seeking ancient area lore/books, meeting local wise men, and summoning related spirits. For a Magical site, a season of practical research, as a Seasonal Activity and with an InVi Lab Total, might work.
A recent one I designed was fairly straightforward - all you need to do is touch the gate and say the name Astaroth. And I had the door give any character that touches it a vision of Astaroth and likely make him say his name, so....
- Anyone who can see the next level of a regio can lead any number of people into it. (AM5, p. 189)
If you can see the next level thanks to spells or abilities (Second Sight), then traveling to that level is effortless. The difficulty comes about when you have a level that is sealed. How a level of a regio is sealed can vary from the nature of the regio, manipulation from things like Hermetic Archeology, to possibly a spell.
I would usually leave it to gossip and research. There may be old stories being told, or it may be recorded in letters and books. They might also find a guide.
The source books seem a little contradictory on what is possible. Piercing the Veil suggests that you can always see into the regio and learn how to cross. Page 189 says that, for some regione, you need to walk the right path to have any chance to see anything at all.
Really, though, I think it is valid to design a regio where Piercing the Veil tells you how to enter even if it is not possible at the moment, and one where Piercing the Veil tells you nothing at all, unless it is the right time. It seem absolutely clear that regione should be so varied as to constantly mystify the players ...
In my opinion you have hit upon one of the great wonders of Ars magica. Regiones are mysterious and often rewarding to deal with, both from an in and out of game perspective.
A regio that cannot be accessed save by those with the cunning and fortitude to figure out its nature is a great prize, even if it were to contain no treasures at all.
As @loke has said it, regiones should be a mystery, some regiones should fit neatly into hermetic theory, being easy to analyze with spells such as piercing the veil. Probably some regiones are so only in theory as their boundaries are easy to find and can always be crossed. Such a regio would be almost indistinguishable from a normal aura though. In fact I would go so far as to argue that magic should be mysterious in ArM, as it fits the theme better for magic to be well magical, as opposed to a science.
Some regiones should be a complete mystery and some be completely straightforward with everything in between being possibilities.
IMO magic should not be so easy as to allow everything to be solved by casting a spell. A wizard ought to need to research and guess and find clues to truly prosper. There are many ways that a regio might give clues as to how to enter or even to its existence. Trial and error is another possibility. Think of Gandalf's attempt to open the Doors of Durin, where his strategy is to try out different common magical approaches and to study the doors because they might give a clue on how to open them. To me this is a perfect example of how a magus might go about trying to find ways into a regio with an unknown entry method.
I personally think there is plenty of evidence in canon of regiones that are invisible to Second sight and Piercing the veil, as there are canonical covenants in regiones where it is stated that the magi in the covenant theorize about the existence of extra layers to their regio. Such would make little sense if they could easily prove (and thus disprove) the existence of such layers by casting a spell.
Back in 4ed, we ended up in the lab for a season inventing a specific spell to enter a particular regio safely. It seemed like the easiest way to be able to come and go as we pleased.