How Do You Know You're Ready?

Like other posters say, you won’t know for certain until you actually try since mods, gear levels, abilities, and RNG all affect what % a team can actually managed. There are many routes to HAAT, but

With that being said, here is how you know if you are close:
1) your guild can clear nAAT in a couple hours
2) you have at least 2-4 x CLS in the guild
3) 4 x zFinn resistance teams that are g9+; BB-8 resistance teams are worth 2 x normal resistance teams.

If you do not have any CLS, you will need at least one zKylo to clear P1 and around 8 x chirpathrawns for P3 to make up the difference

Without awesome mods you can expect the following percentage for planning:
Phase2:
Zader (g10/g11) - 4-5%
Zader/Zid (g10/g11) - 5-6%
ZFinn Resistance (g8) - 8%
zFinn Res (g10/g11) - 15-20%

Phase 3:
Chirpatine -5%
Chirpathrawn - 10%
CLS, Jyn, Old Ben, St Han, Chirrut - 15-20%
CLS, Raid Han, R2-D2, Jyn, Chirrut - 20-30%

Phase 4:
Wiggs rebels - 5%
Wiggs, CLS, R2, Leia - 25%
Res w/ Bb-8 - 75-100%

There are plenty of other ways to HAAT, but to take the homegrown approach will take a ton of resistance teams, thrawns, and/or CLS to get there faster.

Test it out.

But if we test it out and can't, we just wanted today we could've used for an easier one to get gear

Some website like crouching rancor has a readiness guide .

unfortunately the best way to find out is to "waste" the 90k tickets and try a HAAT

we tried it once and got halfway into P2, knew we needed to focus on gearing up most of our squads then
we tried again 6 weeks later and nearly beat it (20% left of P4) mostly due to time zone differences in the last push.
we then cleared it a week later without mercs and now have it on a comfortable farm.

the best way to know how much work your squads need is to do it...

I think like a lot of people have said, step 1 is to just jump in there and try it.

So far as I can recall, if you're finishing the regular AAT raid within a couple of hours, it's a good indication that you're at the threshold of readiness.
Some factors will then make a difference - HAAT is a G10+ raid, so do keep this in mind when assessing your team viability. There is an exception to that, you can make huge gains in phases 2 and 4 with a G9 Resistance team with Finn zeta.

Co-ordination is also a big deal when you're trying it out for the first time. Does everyone have an idea of which team they're going to use for each phase? Is everyone around on the day or night you plan to launch it? Maybe schedule it a week ahead or so to give people time to prepare for it or at least let you know if they can't make it.
When you have attempted it, assess what worked at what didn't. Ask every guildie to post a screenshot of the eventual team they used for each given phase they attacked. Ideally you'll have 50 screenshots x 4 phases worth of useful data there. That's the biggest learning exercise you'll ever have - because every member will collectively get a feel for the teams that are working, and those which aren't, and where they need to improve.

One way of looking at the HAAT is that each of your 50 members need only do 2% damage per phase. Which looks like this:
P1 - 86k
P2 - 384k
P3 - 240k
P4 - 240k
So for players who can't achieve that amount in a given phase, you can discuss, should they work on improving say their P2 team, or if they're doing really well on P3, maybe improve that team even further?
In reality, you'll have big hitters and not so big hitters in the guild, so the overall damage will be skewed, some will achieve 10% of its overall damage at the first time of asking.

Finally, the last thing you need to know is this: once you've defeated HAAT for the first time, you never look back. It gets easier and easier. The very first time I tasted victory, our guild required almost the full 48 hours to beat it the first time we did it (I reckon maybe an hour to spare). But within a very short space of time, we had this down to 24 hours, and before we knew it, we had it down to 2 hours. It's incredible how good you get at it with practise.

Now, a few months down the line, if given the chance I'd feel confident about solo'ing it. It's great to get to the point where your teams have become so powerful as this, so, enjoy the ride!

I moved into a much better guild after struggling through an NAAT over two weeks in my old guild. The best advice I have when moving up is assign guild members specific phases. The last thing you want is to waste damage in the raid when you're first trying to beat it.