[Xara]

The biggest giveaway to mech play is multiple factories with no additional barracks. In general, whichever building Terran has the most of is what they're playing on, Barracks being bio, Factories being mech, Starports being sky (though I'm usually the only one I ever see in my ladders playing Sky Terran). I haven't played that much in recent months, so I can't tell you exactly when you should scout, but keep your scouting active and do your best to spot what they're doing.

The biggest flaw to Terran mech is immobility, and that is one huge flaw in this game, bigger than it was in Brood War. I like to make the very general World Wars comparison, where the first game (Brood War) was a game of power and positioning, and the second (Wings of Liberty) is a game of movement and attacking.
The Terran mech army is without question the most powerful and cost-efficient army in this game once it's set up, so I rarely like fighting it head-on unless I already know I have an overwhelming advantage. What it also means is that Terran mech armies have the tendency to play NaDa's Tornado Terran style, macro up while harassing, keeping you from attacking until they're 200/200, and then move out a 200 supply army with the knowledge that it can and will crush anything that tries to stand directly against it. Once they move out your proactive scouting and Overlord placement should give you a general sense of where their army is at all times.

Against it, you can do any of the following:
1) Once you see them move out, you can stick all sorts of weird and wonderful things up the rear end of their base: Mutalisk, Nydus, doom drop, backdoor aerial siege, you name it, even a frontdoor drive-by if you see it isn't defended. You do this to an undefended base and mech immobility means Terran is suddenly playing an all-in attack because they can't get back in time to deal with your attack before they lose their whole base, and you can withdraw any time you want if you need the units to defend or even backflank the impending attack.
2) Race your tech to BroodLord and take advantage of the combination of Terran mech's lack of mobility and lack of long-range anti-air to out-siege them.
3) Use burrowed Roaches to your advantage. Siege Tanks do a lot of friendly splash, so sticking any tanky unit into their lines without them knowing causes a massive amount of chaos. Be aware that Roaches are classified as Armored, so in a straight-up fight Sieged Tanks do melt through them like a hot knife through butter. I wouldn't advise using Roaches without Burrow and Tunneling Claws.
4) If you find that they're not using the terrain to their advantage like they should, you can out-position them by using high-ground tactics and LoS manipulation. The core units of Terran's mech army are all armored, which makes Neural Siege Tanks or Thors a very good support unit if you can get ahold of one. Be sure that they don't have any air units spotting for them before you try this.
5) The damage output of Terran mech is large single-target burst with some splash. If you don't think they're building as many Hellions as they should, you can consider adding a mass of Zerglings to your army.
6) Spend most of the early-game expanding like a madman and build a few harassment units. After all, you have 5-Range 175HP Queens and they can't move their stuff out the front without you putting your stuff in the rear, so what can they do about it?

Units-wise:
-Zerglings are decent if they don't have enough splash damage in their composition and can be used as meatshields for the incoming army, but shouldn't be your entire army.
-Roaches should be your main ground unit, and you should use Burrowed movement to its fullest.
-Infestors are very situational, and should only be used if you feel you have a tactical advantage. You need to be able to use Infested Terran and Neural Parasite without being punished for it. If you do have that advantage, Infested Eggs can make good siege breakers.
-Mutalisks are an amazing harassment unit, and don't just use them for mineral lines, you can use them to pick off any stray Tanks you catch out of position. Any units you kill now will make that deathball that much less intimidating when they finally decide to attack. Make sure to utilitze the magic box trick for those Thors.
-Corruptors should only be used if they compliment their mech with heavy air. You shouldn't need too much of this so it's advisable to morph most of them into BroodLords when you have the tech.
-BroodLords will be your lategame/endgame. Terran in general doesn't have a lot of long range GTA (Ground-to-Air), so BroodLords and their ATG siege-breaker style means the immobile Terran mech will have a very hard time dealing with this, and you can use them to end the game by pushing them slowly into their base, using their own siege style against them.