The Tor Project tells relay operaters to set a value called MyFamily to declare which relays are run by the same person or group, so that Tor users won't use more than one relay in the same path with the same operator.
https://2019.www.torproject.org/docs/tor-manual.html.en#MyFa...
Of course, a malicious relay operator that wanted to increase its chances of being used as both the entry and exit nodes in a single path (and thereby easily being able to correlate traffic between its origin and destination) could add a lot of nodes and not own up to their relationship.
Some people in the Tor community try to watch for relay-creation behavior that they consider suspicious. A common example is a large number of new relays that appear within a short period of time with similar characteristics and don't declare common ownership.
Edit: one of the main tools for this is OrNetRadar, which seems to primarily use the autonomous system number in which the relays are located, as well as the timing of their creation: https://nusenu.github.io/OrNetRadar/
In this case, the relays can be given a flag like BadExit by the Tor developers, which will stop any Tor client from selecting those relays as exit nodes in a path. However, a sufficiently malicious attacker could add a lot of network capacity in a way that isn't recognizably associated as belonging to the same entity, for example by adding nodes in different data centers, with different speeds, with different software environments, and not all coming online at the same moment. In that case, there wouldn't be a way to easily infer that these nodes are associated with the same operator.
As someone has said elsewhere in this thread, there's still the hope that if different organizations add network capacity with malicious intent, they tend to undermine one another's chances of succeeding, at least as long as they aren't directly colluding (because the basic security goal in Tor is that clients choose paths whose constituent relays don't share information with one another).