I do not consider myself that 'smart', but here are my random thoughts:
You could generate some generic tests for some of the add ons.
Today all accounts have access to upgrades through Stygian Abyss - so this can be assumed as the current base.
Several items below can be done with a new character: Numbers 1, 2, 5 & 6. #3 needs a house and validation of High Seas which grants some additional house storage.
Start a new character with Tinkering in New Haven on a shard you do not play. Buy some ingots (you will already have 50, buying another 125 gives you plenty to test out #2 by separating these). It also allows you to craft tools to test out several of the options.
1) Time of Legends: Test public moongate to travel to Eodon (if no ToL then the option isn't offered)
2) Bank Storage: add non-stacked quantity to your bank until it reaches it's limit -> count all things in the bank
3) House Storage: house accessible character could access the house sign and via OCR determine the house storage options
4) (6th??)/7th Character Slot: At character login/creation page - count how many characters are defined, if 6 or 7. I think ML allowed 6 characters and all accounts that are non Endless Journey are allowed to create up to 6. The only add on these days is the 7th character slot, though "6th slot" was one in the past. If 6 characters already exist on the account, then the "New [character]" button would confirm 7th character slot ability.
5) Gothic and Rustic add-ons have some overlap, so it is more complicated:
Gothic - allows some house gothic items (stained windows), crafting a gargoyle statue
Rustic - allows crafting some items related to fermenting which needs the tinker/carp tool to see if these are allowed or not
I think these overlap in use of the chest of sending and owning a Magincia vendor plot-if I am wrong then maybe these could be valid tests.
6) High Seas - I can think of 2 free/cheap possibilities:
6-1) Does crafting cannons, cannonballs (blacksmithy) , making charges (alchemy?) or their ingredients require High Seas? (my guess is that it does, so this is a cheap option that requires a crafting tool)
6-2) Will the fish monger grant you a Professional Fishing Quest? Without having runes, runebook, crystal teleporter, the easiest to travel to and create a rail is probably Moonglow, while crystal teleporter make Britain a really short rail from the bank.
There are several options on HS but most take an investment of time or maybe some gold:
a) mine up saltpeter (needs luck suit and dungeon mining and GM mining)
b) fishing up HS only fish: Needs a lot of time fishing or fishing skill > 50 to quickly determine if you only get classic fish. Until you fishing is high enough, you can still get the classic fish which doesn't prove anything.
c) tossing a white net gets Osiredon (not only Leviathans): Just way too much effort to determine if you have High Seas
d) sacrifice a map at the Covetous brazier (or whatever it is named) to lead to summoning Corgul
e) Can you place 2 boats? Even the cheap classic boats are limited on a HS upgraded account, where non-EJ/non-HS can place multiple boats.
I can't think of any other options, but I am sure a test could be made in game for most.
There are excessive tests, like having both an add on and it requires a recipe to craft - so you could get false-positive results (If I have the add on (Mondain's Legacy (technically everyone has it already) and Time of Legends both have these requirements) but I don't have the recipe can look the same as not having the add on).
Gaderian
