You can still sweat at a McDojo/McDojang. The instruction may be lax, but it doesn't mean that people still don't get a good workout...but, by the same token, they may not be learning any or all of the art.
Prices for everything should be the biggest thing to watch out for...it's one thing to require that everyone own every piece of sparring equipment made...it's another to requre that everyone purchase it only through the dojo/dojang.
You shouldn't have to pay a testing fee when you're not testing for the next full rank...sure, you can test for stripes on your belt, and I understand their value....but it shouldn't cost anything to test for a couple of strips of electrical tape on the end of your belt.
The price that you pay per month shouldn't be more expensive than your average powerbill at home...if it costs more to train at a dojo than it does to have lights on at your house, then you should probably rethink your priorities...unless, of course, you have a small, energy effecient home...in which case, you suck for being able to save that much money.
Sometimes, family package deals are a good idea, sometimes they're not...I think the family package deals I would watch out for are the ones that have the entire family testing for ranks at the same time...look around and see how many families you see in the dojo/dojang that all have the same rank, especially the higher rank ones. You shouldn't see an entire family of redbelts...usually people take different lengths of time to earn higher ranks. Little Billy may take 6 months to go from Blue Belt to Red Belt, while it takes Daddy only 2 months.
Watch out for a large number of blackbelts...not that a single dojo/dojang can only have a certain number, but remember that a blackbelt should be the hardest belt to earn.
Also, watch out for wee little blackbelts...typically under 10 years old.
If you sit in and watch a class, pay close attention to the blackbelts in the class, if there are any. Blackbelts can be out of shape, but you should be able to tell why they earned their blackbelt initially. Unless there's something physically limiting the blackbelt that prohibits movement, their technique had better be lightyears beyond the whitebelts.
Count how many belts there are between white and blackbelt. More than 6? It's been done with an extra belt or 2 thrown in...but more than 8 belts...yeah, something's not right.
The typical progression of rank in TKD is white, yellow, green, blue, red, black. There are variations in there, especially when we're talking about different arts. But, when you start seeing camo, tie-dye, rainbow, or any other combination of colors, be wary.
The instructor had better be able to perform the techniques...note that I'm saying instructor here...not the person actually "running" the school...but the person on the matt that's giving the directions. If they can't kick and punch like the students can....then you may want to start walking away.
Credentials is another thing...many instructors have quite a few certificates and things hanging on the wall. Feel free to inspect any of them, and remember what the certs are for so that you can research them later...or, better yet, ask about them. And then research them. Just because an instructor has 17 plaques on the wall doesn't mean that he's any more qualified to instruct than any of the whitebelts in class.
So, yeah, that's my list that I usually keep in mind. I'm sure I'm missing some things in there.