We know very little details about Jorge Alvares expedition, in fact even the year was slightly uncertain and the conses shifted to 1513 from previously thought year of 1514.

Basically, the single reference to this voyage by Barros mentions only Alvares being in China and burying his son on the island of Tamão (Tunmen), while another reference by an italian named Andrea Corsali casually mentions in a brief excerpt that some Portuguese (with no names given) reached China (in a year before 1515), weren't permitted to land, but managed to conduct some trade and earn some profit, and confirm it's a rich country. This two references as you realize aren't much to reconstruct any details.

This lack of information prevents us from making any hard claims on what did he Alvares know, or do in China and how did he even get there. We know Alvares didn't go directly from Europe, but actually started the expedition from Malacca, an important port on Malayan peninsula which Portuguese took in 1511.

Further, most works mentioning the expedition claim that Alvares and other Portuguese made the trip in a junk - meaning a local ship (some go even further and claim it was specifically a junk from Pegu, but I can't find a confirmation on it) which would also make the travel much easier.

This shouldn't surprise us, as Portuguese then were often traveling with locals in their native ships, in part due to there being only a few Portuguese ships in area with their own tasks, but more importantly because the locals were regularly making this journey and knew the way and customs of china and everything necessary for a successful voyage.

This ties into your question: "How well did he know how to get to China to sea?" Well, we can't be sure as he didn't leave any remarks, but all Portuguese had to do was ask the locals and hopefully receive an honest answer and they would know enough.

The descriptions Portugese made of Asia in years past 1515 (Tome Pires and Duarte Barbosa) roughly paint a nice description of position of China and how to get there and how does the coast line (focused on Canton) look like. they make some understandable mistakes, but nothing serious. Of course as this descriptions were all post 1513, in fact Pires even alludes to Portuguese had just reached China in his work, it is possible his work is made on basis of Portuguese accounts and not locals, and it does not helps us answer what could 1513 first expedition think.

To sum up, we have no records of what could Jorge Alvares had known before going to China. Portuguese could learn a lot about China while questioning any of the many local traders and pilots who've been there and get relatively accurate idea. They could also - and they most probably did exactly that - go with these local traders in their junks and travel to China together