I don't know why these scams still exist because they can't have a very high success rate. They never worked on me but I came over here with my guard up pretty tight to the point where I may have actually missed out on meeting some cool people because of it. There are some really common ones that are used mostly in high tourism areas that you need to be aware of and they are easy to identify and avoid.
The "museum / tourist attraction" is closed today scam
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This first happened to me at the Grand Palace in Bangkok, which is one of the most heavily visited places in the entire country and with good reason because it is an impressive place. In every direction around this place though, there are scam artists that are lying in wait. Unfortunately their tactic of approaching you and pretending to be helpful is exactly what gives them away. There are some - a lot actually - genuinely friendly and kind outgoing Thais out there but if you are near a tourist attraction there is a 99% chance that they are scammers. The police don't do anything about this because it would be probably impossible for them to do so since these scammers do not approach any Thai people.
The prey on people's friendliness and unfortunately, this is the reason why a lot of tourists completely tune out anyone who speaks to them in public after a while. For my group, we were heading to the Grand Palace but didn't know exactly how to get there. All we could tell was that it was across the way from this park that we were in and like clockwork, here came the "helpful strangers" who just happen to be hanging out in that park. They go through the usual where are you from nonsense even though they do not care where you are from. it's a scam. At some point in the conversation they are going to ask where you are going and the answer is always the same that the palace, or whatever thing you were planning on going to is closed for repairs or something like that. Then they try to lure you into some trap that is going to end up being a gem shop, a tailor, or some other something or other that you have no interest in. Also there are fake ticket sellers around the palace. Do not buy these tickets. Even if it takes you a little longer than you would like find the official ticket booth to this or any other attraction in the country. For the most part they are the only ones who are legally allowed to sell access so if you buy one of the other tickets you will not only lose money but will not be able to get into the palace. The palace authorities will not care that you were scammed either.
The "one stop" taxi scam
I haven't heard about this one happening so much anymore because apps like Grab, which is the same thing as Uber and Lyft, have become so popular here that it is no longer necessary to negotiate with taxis to any destination. However, people still like to ride in tuktuks when they are visiting Bangkok, i guess so they can put it on their FB page.
Get yourself a tuk tuk one time while you are here and then probably just never do it again. There is really no reason to ever use this mode of transport. It is not faster than a car or motorbike, it is loud as hell, and you are breathing in the fumes of the very vehicle you are riding while you are on it. It's fun the first time you do it but there is a reason why these things have been disappearing from places like Chiang Mai where I live because we don't need them for anything other than tourists who want the "real" Thailand experience even though Thai people do not use this method of transport.
If you are ever told by a tuk tuk driver that you are talking with about a destination and he says "one stop" walk away right away. That one stop is going to be a high pressure sales area like a tailor or gem store and while that isn't the end of the line because you do not need to actually buy anything it is a very uncomfortable situation and it also takes a bunch of time out of your day. The one time I got scammed by the "one stop" tuk tuk people I ended up angry and initially the tuk tuk driver refused to take me to my original destination. I took some pictures of him and his license and just said "police" and he reluctantly and very rudely dropped me off where he said he would originally. The fake friendly nature of this person disappeared immediately once it was clear I wasn't going to fall for it.
The "join us for beers" scam
This one doesn't happen nearly as much as it used to because of review sites but it does still exist. I'm not saying to never join someone's table for beers but if at any point they attempt to pressure you to go with him, her, or them to their next destination at an authentic local restaurant of basically whatever it is, refuse. Just stay where you are. There is no reason for you to go with them to this restaurant. A lot of times when the con thinks that they have got you hooked, they will offer to pay for the drinks that you have already had at the one place to lure you into a false sense of security. What happens next is they will take you to a restaurant that has no menu in English and they will "kindly" offer to order for the table without ever mentioning how much anything costs or who is going to pay for it. About 3/4 of the way through the meal your friendly stranger is going to use the bathroom and never come back. Then you will be approached by several members of the staff with a bill that is extortionately high. You could run away to get the police I suppose, but they are prepared for this and the location is not near a police station anyway. You would just have to get lucky and have a cop just happen to be outside and even then, the police is very likely to side with the restaurant owner anyway.
it's unfortunate but this does happen quite a lot. I hate that it is like this because most of it just makes you want to avert your eyes and never talk to strangers.
These 3 things are easy to identify though and while they are much more likely to happen in Bangkok than anywhere else, these sorts of scams exist anywhere in the country where there is tourist activity.