On bus Ethernet, how a connected station detect link connectivity?
In 10BASE5 and 10BASE2, a link relation (between whom?) cannot be resolved at the physical layer because there is no physical (i.e. "inside" the corresponding IEEE 802.3 Physical sublayer) link ("logical connection/association") at all here, despite the presence of a link ("hardwired connection"), typically 1:n as it is a bus, at the medium (i.e. by the coax itself).  Therefore, a special protocol operating over the MAC sublayer was designed for, including but not limited to, resolve such a link. It was the Logical Link Control (LLC) protocol forgotten later as TCP/IP became popular.

In other words, in 10BASE5 and 10BASE2 you can state that a local station is linked with a distant station only by a higher layer association activated/supported, e.g. a TCP session established between the stations. At the physical layer itself, a station can observe only packet activity/collision: this is CDMA/CD in its pure "reincarnation".

It seems logical because on a point-to-point Ethernet, a link shows only that a station is already connected to a distant, compatible physical layer, knowing nothing about what is it: a switch, a terminal device, or a test loop, i.e. the (physical) link itself does not make you informed about whose MAC or higher (IP, socket) entities are your counterparts in this topology (virtual bus) too and to resolve are the selected stations linked or not, a higher layer association (identified by at least pair of MAC/IP addresses) is also needed.