I agree with the isInGods approach, but I think if I was coding something like this, I'd make it even more explicit -- something like: isMemberOf to show that the method was explicitly checking membership. Then again, I don't really know the details of how it's being used. It's probably a very specific case that doesn't need to be support multiple groups.

As for aliases, I think when used in moderation, perhaps it could be valuable, but it's a practice that can easily get out of hand, and add to the burden of someone else who is reading the code.

In Tk, for example, when creating your own custom widgets, it's possible to create many different aliases for option names. These aliases usually are a shorter form of the option name (-background becomes -bg), but that isn't always the case. Sometimes they are different names entirely (-choices or -items). There are different reasons why this is done, but in the end it means that there are more different names that basically mean the same thing, so that someone reading the code has to be aware of both, and may wonder are these two calls/options really the same thing, or are they subtly different? I think the same would be true with subs.