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One technique my professor said is that fake/scam conferences are 1-3 days of programs, while the real ones are at least 4 days and more.
Another technique is to check if they advertise accommodation facilities on their webpage.
Another technique is to check if they list many conferences on one website.
What other techniques can I use to spot fake/scam conferences?
The criteria you mention are unfortunately going to cut off many valid conferences and workshops as well.
It seems to me you are interested in filtering out the bad conferences. This requires a tremendous amount of work or a lot of experience. If you have time, you can spend it in a more meaningful way and if you do not have the experience try to stand on the shoulders of some "giants".
As a first step, think about why conferences exist. It is for people to meet and discuss/show their results. But the conference is only a venue. If you receive a personal invitation to attend a conference, from the conference organizers, an invitation that flatters you and is connected to a specific work of yours, something that makes you think "this conference really want me to participate there" you can be 100% sure that the conference is a scam. Because the focus is on you, while serious conferences try to put the accent on the "exchange ideas with good peers" part, not on the "we give you a good venue to present your results". It boils down to reading "the intentions of the conference organizers".
As a second step it is best for you to focus on some reliable conference (for example the one attended by your advisor, or the very local ones) ... once you start to know the names of your relevant peers, you will see that the reliable conferences are the one where 50% or more of your peers shows up.
This may filter out some very interesting and new conference ... but since the goal of a conference is to have interactions with your peers, pick them accordingly to what you want to discuss with the people presenting something (as a non-exhaustive list of things worthwhile to discuss in person are things like your results, their results, new approaches, future career jumps, finding someone presenting data relevant for your research).
In short: a conference must be of your interest to attend. If you are no one in science and you receive a personal invitations to present your results at a conference, that is a 100% scam.
This is tricky. It takes some background knowledge and experience to spot "scam" conferences. You also have to decide what you mean by "fake/scam". There are plenty of sketchy, poor quality conferences that are not, strictly speaking, fake. They exist. (Some) People show up to them. Talks are given etc... But they are not considered quality and they exist to make money, not serve as a forum for scientific discussion.
There are probably straight up fake conferences out there, but I think (correct me if I am wrong) you are more interested in those "predatory" conferences that technically happen but are a waste of time and money.
I usually use a few checkpoints to decide if a conference is sketchy:
There should be enough obviously reputable conferences ranging from major events hosted by national or international organizations to small local meetings hosted by chapters of professional societies or universities that you should never really need to spend time evaluating a questionable conference.
