Kill Silverfish

Not every Igloo will have a basement. You can tell if you have an igloo basement by checking your pie chart for a sign, and can find the igloo by pieraying that sign. (Some Taiga village houses can generate with signs, so this is not foolproof.)

Stronghold infested blocks are not uniform like they are in Igloo Basements. The only infested blocks that can generate in strongholds are Infested Stone Bricks, meaning that Cracked Stone Bricks and Mossy Stone Bricks can only be infested if an existing silverfish burrows. Of the stone bricks themselves, about 1 in 10 will be Infested, which you can check quite quickly by mining or using F3's Targeted Block. Blowing up a room with a creeper/TNT is also very quick for checking lots of blocks. (This is assuming that you found the stronghold from the Starter Staircase, in which case you don't have easy access to the silverfish spawner, but if you do, you can of course just kill a silverfish there.)

Mansions are rare structures and are often irrelevant for Draftout, but conveniently the only room that can possibly have infested blocks is also the only generated structure in the game to have a trapped chest. As long as the mansion is in your render distance, you can use the pie chart to see if you have a Fake End Portal Room by looking for a trapped chest. This room is a secret room, but if generated it will always be on the first floor of the Mansion. You can also get 2 TNT and 2 ender pearls guaranteed from this room. 

When a Trial Chamber generates, it picks 1 mob per category for every Trial Spawner in that category. Silverfish are 1 of 4 in the 'small melee' category, meaning every TC has a 25% chance that it can have Silverfish. The silverfish trial spawner has 4 ordinary stone bricks (not infested) in the corners (see image above). In Trial Chambers, guaranteed rooms include:

(a) the 8 corridor slices (4 x 2 corridors), 

(b) the chamber connecting to the end room, 

(c) and the chamber connecting to the intersection. 

The chance for you to get at least 1 Small Melee trial spawner in both corridors is 77.412%. If you see a Slime, Baby Zombie, or Cave Spider spawner, you know that your trial chamber won't have silverfish. 

The chamber connecting to the end room can only have silverfish if it is Assembly (see image below). If you have Assembly there is a 21% chance it has a small melee spawner. None of the other 3 End Room chambers can have silverfish spawners. 

The chamber connecting to the intersection can be Assembly (as above), Pedestal (55.56% for a small melee) or Chamber_4 (the 'Small Melee room', which has a 25% chance of having Silverfish). None of the other 5 chambers can have silverfish spawners. (See below as well for both of these.)

This means the chance of getting a Silverfish Spawner in a Trial Chamber is roughly 20.83%, not counting the side rooms from copper doors. 

If you whiff, you can still try to force a lingering potion of Infested by triggering an Ominous Trial. Ominous trial spawners choose a random lingering potion out of 7 options when activated and use that lingering potion throughout, one of which is infested. If you get one of the other effects (Wind Charged, Oozing, Weaving, Strength, Speed, Slow Falling), you can move onto the next Trial Spawner. This choice is location based, so Trial Spawners close together often choose the same potion. 

Infested stone and deepslate can generate as ore blobs in mountainous biomes (in fact, exactly the same biomes as emerald ore) from y level -64 to 63, with the highest concentration generally being around y = 0. (The exact biomes are Windswept Hills, Windswept Gravelly Hills, Windswept Forest, Grove, Cherry Grove, Jagged, Frozen and Stony Peaks, Snowy Slopes, and Meadow.) It can generate replacing any stone type but can only itself generate as stone and deepslate. 

The maximum number of blocks per blob is 13. The biggest problem is that it's very unreliable, especially because you need to be within 20 blocks for f3 to pick up on the targeted block. You'll realise if you get to infested stone/deepslate because most of the nbt tags from stone or deepslate will disappear. You could blow up blocks with creepers or TNT to break more blocks, though being air exposed doesn't matter to infested blocks. There will be infested blocks, but finding them is time consuming. 

This could be a fallback option if your Trial Chamber whiffed and you are in these biomes.