Most of the time it's hard to tell.
However, I will judge by the number of telegram groups, if the popularity of this project is high enough, I think it can be completed by ICO. Generally speaking, the projects that have been raised by ICO will pay you the relevant reward, so I seldom worry about this problem.

It's hard to tell which one is scam or not.
but there are several ways that is
find info about the bounty and check its website as well as the background of the project. and see who the manager who manage it. because quite a lot of managers who have good reputation

It is not so easy to tell if it is a scam or not, what you can do is doing some deep research, including reading the whitepaper carefully and background investigation to the dev team.

What you must do is read the whitepaper and see how professionally presented it is. Is the English clear? Are there common grammatical errors? The most important thing is whether you understand the problem that the project is solving. Make no mistake, it's easy to think that a project sounds too complex for you to "get it", but more often than not with a scam there will just be empty words that sound intelligent but are really meaningless. That is why you must get that "ah hah!" moment when you read the whitepaper, and realize.. "so that's the issue they're solving". After that, research the team, verify their LinkedIn is real (check to see if they really do hold a position in the project according to their profile). Lastly, join Slack/Discord/Telegram and see if you can chat to the team to also verify they're real. All of these steps will help to mitigate the chance of joining a fraudulent bounty campaign.

Even though the topics/articles which I posted below are not directly related to your questions, I believe it is a good resources or discussion for you.
https://bitcointalk.org/index.php?topic=119896.0 | How to Spot a Scammer by Tomatocage
https://bitcointalk.org/index.php?topic=1963768.0 | How to know if the ICO is scam (discussion, sadly it is inactive)
https://www.cryptocompare.com/coins/guides/how-to-avoid-scam-icos/ | Cryptocompare articles on scam ICOs

Regarding your questions, there is something to consider before you join a bounty from my experience, including but not limited to:
- The manager's reputation. If a manager is a newbie or something like that, proceed with extreme caution, unless it is clear that those newbies account are an official account from the team.
- Funds, whether it is escrowed or not. Escrowed bounty is the one you should look, however, there is not many out there.
- The team. Avoid bounty of a project that doesn't have a clear and if possible a great background on cryptos and etc. However, some good coins out there has an anonymous team, so this rule can't be applied to every condition.

Look for the whitepaper and the background of the project,check developers and the founder,product and roadmap.

You are right I have been victimized two times already it's a waste of time and effort for 5 months so it's very important to do a lot of research if the ico is indeed legit, check up the team behind it and the roadmap, it's better if a reputable company will headline the ico.

I think you are referring to longer period social and signature campaign which are not escrowed.
In my personal experience, Joining a campaign under a reputed or professional bounty manager or under a contributed member (copper member and higher) has lower chance of being left without a pay.

Of course, we do not know that this project may not be legal, but on the other hand, there is always a risk, as well as the acquisition of a koin on the stock exchange, because people do not always have time to get acquainted with a coin or a token, more accurately with its legal nature, people are in a hurry to make a purchase based on the news about this project.

Before I'm going bloody research to know if the project is legitimate or not but now I just find it in choosing the good bounty campaign managers because good ICOs and coin company have no funds issue or problems therefore if they want their ICO to be promoted by good  members then they could hire experienced and good bounty campaign managers like Sylon, Needmoney, Arteezy, Yahoo and more.