Old decks of playing cards often don't have corner indicators for suit and number, and it seems hard (at least to a novice) to determine what each card is without seeing it in full. For instance:
This is an Italian deck, but other old ones I see are similar. I normally hold cards more or less like this, with only the corner digit and suit visible:
This does not seem possible with old cards. Were there any techniques for holding a hand in such a deck such that the cards are easily identifiable?
No, there's no easy way to hold the cards so that the cards are identifiable without adjusting them to look at each one's full face.
The solution to that problem was the "major innovation" that led to patents being granted for the corner emblem (indicating the absence of a practical way of holding and viewing the cards prior to that innovation). Source.
A patent was issued on February 9, 1864 to Cyrus W. Saladee of Paducah, Kentucky, under patent number 41,587. It is believed to be the first American deck with corner indices.
My invention consists in combining a number or letter with an emblem - such as a heart, spade, club or diamond - so that upon seeing the emblem, which may be in a corner, the denomination of the card is at once understood.
This idea revolutionized American card manufacture by enabling players to fan the cards to view only the corner of each card, sufficient to see the indices. This was soon followed by Andrew Dougherty’s patent for “Triplicates” and other novel systems of index types which were adopted by other manufacturers until, over time, they became the new norm.
These cards are tall and narrow at 103 mm by 51 mm (4 in by 2 in). So holding them in one hand can be a little awkward. The 40 card deck in the question is a Primiera Bolognese pattern. Alta Carta labels them as L0061 and notes:
This pattern will soon vanish. There is only one manufacturer left that prints such cards. This does not surprise me, because these cards appear very inconvenient to play with.
The book "On Games of Chance" by Girolamo Cardano described Primero with a hand size of four cards:
Two cards are dealt each player
...
After the first round of betting, two more cards are dealt to each player
Corner and edge indices (see history below) enabled using only one hand. I printed some replica cards and held 5 in a hand with relative ease:
The New York Consolidated Card Company began printing cards in 1832. Their patented design in 1875 is credited with popularizing corner markings to allow fanning your cards for better visualization.
It was not uncommon to hold your hand of 4 or 5 cards in two hands at the time:
Corner and edge indices enabled people to hold their cards close together in a fan with one hand (instead of the two hands previously used). [emphasis added]
They were originally called "squeezers" and not well received by customers, but are now referred to as "indices".
I've played several games using Sicilian cards that are the same style of deck. You can reasonably hold 4 or 5 cards in one hand and still see them. You only need 30% - 50% of a card to be visible once your familiar with the deck (you can see the suit and enough of the picture to tell the value). Holding the cards aligned flat in your hand rather than fanning them at angles also helps. If you need to hold more than five cards, you can put some of your cards facedown on the table, or use your other hand.
You could also use wooden stands like the following:
