What he has is more than doable. I'm on a day 25 game with 237 units and 170 cities captured. No gold used. And I've also been lazier than usual in this game.

Not saying he didn't use gold, just that what he has is achievable without it. You don't mention how many cities he has, because relative to that, one can infer if his Grande Armée is kosher on the face of it. If he also blitzkrieged, then he is going to snowball relative to you.

In terms of the dude using gold, if he's in your coalition, this really should be evident vs it being a hunch. Keep an eye on the production timers for things he's producing to see if he gold rushes. Calculate his hourly resource production & look at the economy report on the news, and see if what he's building makes sense vs that. See if he's levelling or has researched more unit types than is reasonable relative to rares production/market buys. Point is, just have some evidence on it instead of an argument from incredulity i.e. "he has too many things, I don't understand how, therefore gold". If you have to go up against a golder, you'd want to know with some degree of certainty that that's the case so you can plan for it.

As to him only having lvl 3 arms. I keep them at lvl 1 for the first couple weeks & instead just focus on mobilisation which allows for aggressive expansion, which yields more. I'd only revert to an arms ind focus if I'm facing a real challenge expanding & need to compensate. In terms of "you can't keep up with him" - Your level 5 arms ind economy is going to pale in comparison to his level 3's if he has an economy boosted though exponentially more occupied cities than you.

In terms of how you play this? Ingratiate yourself to him. Ask him for advice - people like to feel useful. If he comes to regard you as something more than an inconvenience to swat away, he may spare you. Aside from that, you could pre-empt his assumed move by leaving first yourself and seizing an advantage through initiative. Only you can decide, on balance of probabilities, what is more likely to pan out. If you choose the latter, do it at the most opportune moment and leave immediately at day change to give you a full game day to wreak havoc. Find out his time zone and when he's typically active. Ideally you'd want to catch him while he's sleeping. If he's not particularly bright, you could move troops into his territory beforehand. If he has a spark upstairs, you could fabricate a pretext.

You can see the dispersion and composition of his troops ahead of time. So, formulate a battle plan involving engagements where the odds favour you and where you can effectively counter what he has. While he may outnumber you in aggregate, you can likely concentrate force to overwhelm in certain areas. Use saturation attacks. Use the terrain. Find force multiplication where you can. Sabotage his airfields. Spam distractionary runners. Feign attacks to draw him off. Be a virulent nuisance and aim for a quick takedown.

Skill can be indistinguishable from gold use and people often attribute incorrectly. Unless you have good reason to think otherwise, go speak to the guy, you might learn from him.

Well no, not really.
Oh yes.

Well yes, practically in any case. If someone is skilled and also uses gold, the effects could be so effectively blurred as to be indistinguishable. Are you telling me if you were playing mirror universe you, and the you with the goatee used gold once to buy a single load of supplies, you would definitively be able to say that he golded, and it was not an aspect of skill that allowed him to win if he did?

In time, you will eventually come across someone who is so good at this game and does NOT use any gold, that they will have the appearance of using it because they are that skilled, but they are truly few and far between. However, if it walks like a duck and talks like a duck, it's probably a duck. (Not always. I feel the need to say this to address previous comments, despite my opening sentence)

If your fellow coalition member is a gold user, then you'd probably be the best to know outside of the one using it. Especially if you've been active and watching him every day. Believe it or not, just because someone uses gold does not mean they don't want to play on a team. This is where this conversation comes together; this is not about gold use. It's all about whether or not you are coalition or team members with someone who is much more powerful than you, regardless of how they got there. So before assuming that just because he "doesn't need me to win, he can crush me if he wants" does NOT mean he will. I don't see how or why you would make that assumption. Care to explain? Has he been massing troops on your border, despite the visibility to you? If so, what kind and in what amounts?

The researching is often what gives a Golder away. There is physically no way some players can have certain upgrades completed within a certain amount of time without using gold. If you know their exact reasearch makeup its pretty easy to figure out the minimum research time for all of them, as well as how much rare material it would have cost.