Yeah sure. Assuming you are only targeting Lemmy instances (other softwares make this a bit more complicated), A “can interact” with B if:
A hasn’t blocked B
B hasn’t blocked A
Neither A nor B are on allowlist. If either is on allowlist, it must have explicitly added the other one to its allow list (this is very uncommon, the only big instance using allow lists is hexbear.net)
So, to verify this, you could query the Defed Investigator with the instances you care about, one at a time. Only select the softwares you care about (likely only Lemmy) to make the query faster. Say you wanted to verify the compatibility between lemmy.world and sh.itjust.works (just making an example). Go to defed.xyz/check?name=sh.itjust.works&software=lem…
✔ lemmy.world doesn’t appear in the “Instances defederated from sh.itjust.works” (this means .world hasn’t blocked SJW)
✔ lemmy.world doesn’t appear in the “Instances defederated by sh.itjust.works” (this means SJW hasn’t blocked .world)
✔ lemmy.world doesn’t appear among the “Instances not allowing sh.itjust.works” (this means .world isn’t on allowlist or, if it is, it has explicitly allowed SJW. Again, this is very uncommon)
Also make sure the instance you are looking for isn’t among the “Instances that returned errors”, of course.