For generalized aspects in a chart...Moon conjunct the Sun along with Venus squared tightly by Saturn (especially backwards square).
You can't really say well from any one aspect or thing though...it's when you see things piling up on top of each other all confirming the same thing.
For example of marriage, things like 7th lord in 8th, 7th lord in 6th, 8th lord in 7th etc will show troubles in relationship. The dignity of 7th lord being bad will also add to trouble. For women particularly look towards Jupiter as well...and for guys Venus. (I have not tested it enough to be sure whether this still applies for gay women etc, though still seems to hold well IME)
Certain conjunctions, such as Sun with Venus and Moon with Saturn also will tend to cause troubles in a person's relationships for their own particular reasons. But I would never predict "no marriage" from any one of these alone.
For not settling down, the above conjunctions also apply, especially Saturn/Moon. Also 1st lord in 1st, 7th lord in 1st, 11th lord in 12th, interchange between 5th and 7th. As well as destroyed 7th/9th houses through their lords etc.
There's more things to look at too...part of love/marriage, upapada, planet with least degrees in its sign, 7th and 9th divisions relating to relationships/marriage (those last three are from Jyotish astrology) if one really wants to look more finely.
But the above would be a place to start, when you get a whole lot piling on top of each other.
Any one alone isn't really going to do it, they'll just create break ups or issues when they activate.
But when you have a whole bunch of different planets all causing problems is when you will have life-long issues since the person is likely to always be in a period of a planet that's causing some kind of problem in that case.