“They” know such things only if they check for them and write it down.
Condom trace evidence is well known in law enforcement. It would be standard procedure to check for the various types of trace evidence that a condom always leaves.
Besides, there’s all sorts of other DNA transfers that can occur during a sexual assault, and the likelyhood of these transfers is much greater with a gang.
Had she been assaulted using condoms, the police would be able to tell and prove it.
They would possibly even be able to prove the brand of condom.
In the LAX case, the accuser changed her story to add condoms when the DNA evidence didn’t show up, but that story didn’t work, either.
Use of a condom is obvious during a sexual assault exam and tests.