It's difficult, but if you're actually looking for one, better to take a risk and ask for an invite so you can check out how they run things.
What I'd probably look for is 1 to 2 clusters, with a few outbound, a tribe of good players will have contacts and beable to bring in friends so I'd expect some starting late.
Max probably 25 members at this point unless it's premade or the duke likes to pad out his tribe with meatshields.
Check the twstats tribal members page for oda, odd & points, whilst big point growth at this point could mean awesome farming and an inability to offload all res into troops, more likely it's indicative of longterm inefficiency.
ODA: The chances of not hitting a single spike for an active farmer are pretty slim, so either members with no oda are not farming, or they're unreasonably lucky,
ODD: any members who don't have 1 odd are nuts,
Amongst other things, the ability for tribes to work around the pushing rule, use alt accounts [to hide histories] and/or outright cheat by slave/barb spawning means there are literally no guarantees from simply stat checking, the only way you'll ever be certain is by taking a 'risk' and joining one.
Tribal profile: Dukes that take themselves seriously are almost invariably tools.
I think you should go for it!