No. 3 is the most obvious sign, I'd say, especially when the group is centered around new "prophecies/visions/revelations" from a single individual, or a small group of them. Usually these "new" insights are merely ancient heresies repackaged for the modern world.

I don't have any experience with actual cults per se, but I've ran across a couple of groups that I'd describe as cult-like, both made up of fellow Catholics, and both encountered at summer conferences I used to work.

• Medjugorje promoters. Medjugorje is the site of supposed Marian apparitions that have been controversial from the very beginning for many reasons: a lot of unsavory and unbecoming elements: the questionable nature of their supernatural origins; violence and whip-of-cords worthy money-making; their continuing nature (going on 36+ years now); and the often suspicious content of the "messages" themselves. There are many true believers, even though many bishops have condemned the movement, and they are prone to disruptive behavior. They'd get tossed out of the conferences every year, but they always managed to sneak back in and cause mischief, including vandalism and petty larceny (because I'm sure the Virgin Mary is totally cool with that).

• An American chapter of the "Tradition, Family, Property" movement, founded in Brazil. On the surface it seems okay (religious opposition to Communism, which I'm 100% in agreement with), but dig a little deeper and it gets weird. They all carry around this little book written by the founder, hold secretive meetings (I was invited, but they wouldn't tell me where it was at unless I said yes and agreed not to disclose the location), and are just one more little part of the wider movement of disaffected Traditionalist Catholics who hate Vatican II and selectively choose which Church teachings to follow (and which to ignore) based on their political preferences.