Every situationevery dogis different. No two rescues are the same. Thats why assessment is so very, very important before you jump in.
But assess what, exactly? And how?

Its about two things: history (what kind of rescue itll be), and health (how urgent the rescue is).
HISTORY
There are four ways a dog ends up on the street: s/he was born there (feral), s/he was abandoned, s/he got lost, or s/he ran away.
Street-born vs. Abandoned or Lost
Feral dogs have a very different attitude to humans than those whove had families. Chances are an abandoned or lost dog will be easier to approach; theyre used to humans being a source of security, and theyll probably trust you pretty fast. For a street-born, on the other hand, humans are another danger to avoid, like fast-moving cars or fireworks. Theyve seen a lot more abuse, so theyll be wary if not outright scared. (And a fearful dog does not make for an easy rescue.)
Runaways
Dogs purposefully leave behind the safety of a home, even a bad one, for only one reason: survival. A runaway dog is one whos been abused to the point where his/her survival instinct kicked in and told him/her to flee. For obvious reasons, these dogs are often as distrusting of humans as a feral dog (if not more).
What to look for
Lost
Collar, tag, recently brushed coat, cleanliness, clipped nails; signs that a human somewhere cares. Often look disoriented and distressed. Usually easy to approach. Might be neutered/spayed (easy to see in males).
Collar, tag, recently brushed coat, cleanliness, clipped nails; signs that a human somewhere cares. Often look disoriented and distressed. Usually easy to approach. Might be neutered/spayed (easy to see in males).
Abandoned / runaways
Any signs of human involvement wont be recent: coat beginning to look matted, chipped or torn nails, an old collar, or a mark on the coat where a collar used to be. Extremely skinny, or signs of recent massive weight loss: loose skin, head looks disproportionately large (home-raised dogs without street smarts wont find food easily). Visible skin issues, growths or lumps; disease is one of the main reasons dogs are abandoned or neglected. Often look disoriented and distressed. Abandoned dogs will be easier to approach; runaways might be more wary. Might be neutered/spayed (easy to see in males).
Street-born dogs
Thin but rarely skeletal. Theyre survivors, theyre strong and smart. Lots of ticks and/or fleas. Usually visible skin conditions (bald, scaly, or scabbed patches, or near-total hair loss). Usually seem self-assured, like they know where theyre going. Will probably show distress / wariness when approached. Theyll back away, or growl, snap, or bark a warning. Probably wont be neutered/spayed.
Thin but rarely skeletal. Theyre survivors, theyre strong and smart. Lots of ticks and/or fleas. Usually visible skin conditions (bald, scaly, or scabbed patches, or near-total hair loss). Usually seem self-assured, like they know where theyre going. Will probably show distress / wariness when approached. Theyll back away, or growl, snap, or bark a warning. Probably wont be neutered/spayed.
HEALTH
What to look for
What to look for
- How badly malnourished is s/he?
- Does s/he have visible skin issues?
- Watery eyes and/or nose? Nervous tics, trouble walking? (signs of canine distemper)
- Does s/he eat well? Any vomit and/or diarrhea? (signs of parvovirus and/or intestinal infections/parasites)
- Is the dog visibly disabled or hurt? (limping, open wounds, impaired vision/hearing, etc.)
- If the dog is female, do her teats look elongated and swollen? (She might be suckling, and youll want to rescue the puppies, too.)
- For female dogs, check for an enlarged vulva; if shes in heat, rescue urgency increases.
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Now you have all the information you need to plan your approachwhich well get into tomorrow (because, in rescue, B is for Botched).

Thanks for coming by; I hope you found this informative and entertaining. As a disclaimer, let me say Im not an expert, and the only thing that qualifies me to talk about dog rescue is that, in a lifetime of doing it, Ive botched more than my share. Thats the learning Id like to pass on to those interested in rescue or getting involved in it for the first time. And, if you have your own rescue experiences or advice, Id love it if you shared them. A fresh point of view is always welcome.