It appears to be on the website:
Maple Leaf Lounge Access
Air Canada Super Elite and Air Canada Elite
Air Canada Super Elite and Air Canada Elite members as well as their guests are welcome to access the Air Canada Maple Leaf Lounges when holding a valid membership card and a same-day confirmed ticket for travel on Air Canada, Air Canada Jazz, Star Alliance or an Air Canada Codeshare flight.
All of the other access privleges appear to specify
departing flight. I note that for the United RCC, all access privleges appear to specify departing flights only (and, at least in ORD, the sign at the desk in the C concourse says that you have to present your membership card for access).