Realm Type Notification adds colored realm-type (PvP or PvE) information to the cross-realm invite popup. While this doesn't guarantee you won't ever get coalesced to a PvP realm (since you may be sent to a server other than the one the person inviting is from), it should be a decent indicator of what you should expect.
Implementation uses a list of PvP realms from Blizzard's Realm Status page (with spaces removed to match in-game behavior) and checks the realm of the person inviting against that list. I have included lists of both US and EU PvP realms and pick which list based on locale. This method may not work for "enGB" users as the client still returns "enUS" according to Wowpedia; sorry! Feel free to edit the source to point "enUS" to the "EU" list.
If you encounter any PvP realms that do not display as such, please let me know and include the region and how the realm is shown in-game (spacing, capitalization, punctuation, etc).