Is it possible to spot most red flags before entering a relationship?

You can spot the red flags, but you usually don’t, or somehow explain it to yourself, until it starts getting out of hand.
Some people don’t notice it even when after if got out of hand long ago.
Or, which is even worse, notice it and accept it.

All of the reasons you list above.

Many jerks are so skilled at hiding their true side that when the victims find out it’s already too late, and there are more of them than you think. Some are so skilled that even when the victims know they are jerks, they still can’t get out of the relationship because they are bound to the relationship by too many things. It’s hard to get go of a relationship that you care so deeply for even after you know your partner isn’t who you think they are. Or the victims don’t have anything to confirm that their partners are jerks or good people with some bad trails, since the jerks know how to influence people around them and hide their true intention with convincing lies.

And there are people who are so blinded by love or have a strong need to get out of loneliness that they end up loving jerks.

Not to mention masochists too.

There are of course jerks (sociopaths) skilled at concealing their condition, but more often than not those warts are right out there in the open. It should be obvious that love is or certainly can be blind. That’s just how it works, and for those fools thinking themselves immune, I say you’ve just had the great good fortune not to bump into your particular siren.

I’ll tell you what I don’t want. Some may argue and say that just because someone has these traits doesn’t meant they’re a bad person, and that’s true, but this is what is not for me (just a starting list, doesn’t mean this is everything):

Someone who is drinking in bars nightly.
Someone who can’t get through the day or night without a six pack or a bunch of drinks.
Someone who does illegal drugs.
Someone who is broke or jobless for the long term.
Someone who appears to have some behavioral issues, emotional issues or major mental health issues.

Yes, most people demonstrate their negatives early on. Most people, though, especially younger (under 30) are attracted to what they feel accustomed to.

For many people, a dysfunction they have known is like a perverse returning home. Children of abusive parents choose abusive partners, children of alcoholics select alcoholics, children of a philandering parent end up with partners that cheat.

The possibility for growth, for health, for good relationships lies in helping people identify and overcome the dysfunctions they grew up with. It takes work, but it is very rewarding.

It is possible.

But people tend to have past wounds that want healing, which are unhealed because our brains have been protecting us from them with aversions and self-delusions and blind spots and so on. But parts of us still want that healed, and so we feel attracted to people who somehow fit the patterns of our past, but that might let us work on them in a new context that offers intimacy and attention, such as a romantic relationship.

That has the potential to actually succeed and be positive, either through good fortune or by virtue of the people being fairly emotionally healthy and nurturing, and/or from them finding help from good healers.

But the folks you’re concerned about may tend to get into similar relationships with unhealthy people which often leads to relationships that tend to be dysfunctional, co-dependent, abusive and/or destructive, etc., and tend to reinforce and elaborate the negative patterns rather than healing them.

Do you think it’s usually obvious that these jerks are showing signs of their bad behaviors before they get involved with other people?
-> Not to the people you mean, because they have blind spots and feelings of attraction and comfort towards the “jerks”. The “jerks” are generally people who were impacted by similar past situations, though they may have chosen to be perpetrators rather than victims.

Is it possible that most jerks are able to control or hide their bad behavior until after they’ve gotten involved with someone?
-> Yes. Also, the behavior patterns will be brought along by the victims, also often covertly. The subconscious detects the pattern and sends feelings of interest, attraction, familiarity, comfort, fascination, etc., but not what those feelings are really about. That’s why it can transform someone’s entire life to heal their own material, meditate to listen and understand and process their body feelings, and so on.

Or do you think that some people are just too sucked into relationships, whether it’s by love, or lust, or loneliness, that they simply can’t see the signs, or choose to ignore the signs, until they’re already involved with or married to the person?
-> That’s not an “or”. Yes, they do, and the reason tends to be along the lines of what I wrote as answers to the previous parts.

The red flags are usually there from the beginning but not very large or as brightly red as they will become. As they say love is blind.