You can't. The closest you can probably get is
%device%. This might be less exciting than you think: it's just a string variable that holds the name of the most recent triggering device. If there is no triggering device (e.g., because the rule was manually run via "Run Actions" button or you have a trigger that does not involve a specific device, like a periodic trigger), then it will be empty. The same applies to
%value%, another string variable that holds the value of the attribute that triggered the rule. Running one rule from the actions of another rule is different: the rule that was called will populate both of these variables with the device that triggered the other rule in the first place.
But not all hope is lost. Would it work to just check a specific device state? To give a simple example, if one rule is triggered based on a switch turning on and then that rule calls another rule, would it be sufficient in that rule to test whether the switch is on (e.g.,
IF (Switch 1 is on) THEN...)? Of course, this doesn't mean that the switch triggered the rule, just that it's currently in this state, which is why I phased the question the way I did, but sometimes device states can be useful. (It should be noted that you can do a comparison on
%device% with a conditional to see if it matches a certain string, i.e., if that device triggered the rule or something in the chain, but I'd consider that error-prone and bad practice personally.)
Alternatively, it sounds like you might be doing different actions based on different triggering devices. In that case, I'd ask: why keep them in the same rule at all? As people used to say, "rules are free" (and still do say, but this was a bigger thing back when individual rules held far less power than they do now).
If you aren't sure if/how any of the above suggestions could be made to work for you, then I'd suggest providing a specific automation you're trying to accomplish where you had this approach in mind. Someone might be able to provide advice that is both more targeted and more elegant than the general suggestions above.