Source: Me (Cole Wyeth)
Confidence: High
Relevant background: https://en.wikipedia.org/wiki/Ash_Wednesday
Various Christian denominations observe "Ash Wednesday" which includes smudging ash on their foreheads in the shape of a cross (the cross may not always be easy to make out). This tends to be accompanied by fasting, Lenten sacrifices, abstinence etc. - the details will depend on denomination.
Source: SeñorDingDong
Confidence: Mid
Relevant background: https://www.cbsnews.com/news/trucker-accumulates-skin-damage-on-left-side-of-his-face-after-28-years-on-the-road/
The image is from an actual long-time trucker. In a minor case, you might see sunburns or tanning on one side of the face (the closer window side, so perhaps you can also determine what side of the road they drive on indicating geographic location).
Collar/key Necklace -> BDSM
Source: Saira
Confidence: Mid
Background: https://www.etsy.com/ca/market/key_slave_collar (presumably the content shifts a lot since this is just a link to etsy)
There are lots of fancy little key/lock necklaces that probably have nothing to do with this, but there seem to be particular types that indicate membership in the BDSM subculture. I am really not qualified to go into detail here, but it looks like the "lock" necklaces and anklets often have a little ring like this (the one in the photo is a more subtle example, some look.. draconian). The keys seem to be a specialized cylindrical tool not an ordinary key - so if you see someone with a fancy ornate key it's probably for something else. Would appreciate mechanical and interpretive details from anyone in the know, but will probably leave them in the comments to keep the list itself reasonably SFW.
Pinneapple Symbol -> Swinger
Source: Saira
Confidence: Mid-High
Relevant Background: https://www.cruisecritic.com/articles/upside-down-pineapple-cruise-ships
A pinneapple can also signify hospitality, but particularly if its upside-down or accompanied with other hints, it's very likely an indication of swinging (consensual non-monogamy).
Source: @jenn
Confidence: Mid
Background: https://www.gsmotorcycletours.com/post/motorcycle-nod-or-wave-the-facts
This seems to be most common on the road, but according to Jenn may be used under other circumstances (I haven't directly verified this, but it tracks). It is a kind of solidarity gesture, 2 fingers (= index + middle) for 2 wheels on the road, and since it's done in passing as pictured above it should always be the left hand.
Burnt Spoons/Ash Smudges -> Opioid UseSource: Anon
Confidence: Mid-High
Background: https://oasas.ny.gov/opioids (seriously this link is a treasure trove)
The link has an extensive list of signs of opioid use, including burnt spoons and bottle caps, missing shoelaces and or belts (why? someone fill me in), and sleeves concealing track marks. In addition to these, apparently ash can get smudged on clothes/hands/face.
Abdominal Swelling -> Portal Hypertension (Alcohol Use?)
Source: Ames
Confidence: Low
Background: https://www.highfocuscenters.com/does-alcohol-cause-bloating/, https://www.mayoclinic.org/diseases-conditions/alcoholic-hepatitis/symptoms-causes/syc-20351388, https://www.hopkinsmedicine.org/health/conditions-and-diseases/portal-hypertension
The pictured abdominal swelling looks a lot different than normal fat; it results from liver damage (check the third link for details from someone with actual medical credentials). This can be caused by alcohol abuse. Note (first link) that alcohol can also cause swelling and weight gain as well in the short term, which is a different matter. The second link contains more (often observable) symptoms of liver failure from alcohol, including jaundice (yellowing) of the skin.
Woman Rubbing or Resting Hands on Belly -> PregnantSource: Novy
Confidence: Mid
Background: https://www.reddit.com/r/BabyBumps/comments/4aofuc/holding_resting_hands_on_or_rubbing_your_own_belly/, https://www.reddit.com/r/NewParents/comments/msmpof/anybody_else_finds_themselves_rocking_their_body/
Of course when a pregnancy is far along it's obvious. However, it seems to be pretty common for women to frequently touch their bump for various reasons, including because of discomfort and to signal that it is pregnancy and not weight gain. This may be an early sign.
Also, apparently new parents tend to rock even when they aren't carrying their baby! See the second link.
Dust on Clothes -> Working with or in Drywall / Paint / Concrete / Mud
Source: Ames
Confidence: Low-Mid
Background: https://www.reddit.com/r/Construction/comments/dnstgm/how_do_i_clean_cement_dust_from_clothes/
It's often possible to narrow down a blue collar worker's profession based on the dust/residue on their clothing, particularly work pants (the ones in the pictures are cement stains, perhaps from mixing concrete). Personally, I have a bit of trouble distinguishing the source (e.g. drywall v.s. concrete), but it's probably obvious for anyone with direct experience. Ames summarizes the differences as follows:
Drywall dust is chalky
Paint is rubbery
Concrete is more dark
Mud is brown
Obviously, paint also comes in a lot of different colors.