MLM/direct sales is an old an honorable industry - but that doesn't mean every company in it does a good or honorable job! There are good movies/bad movies, good restaurants/bad restaurants, honest insurance agents/bogus insurance agents. Fuller Brush is an old company, Tupperware and Avon started in direct sales, Mary Kay is well known.
If you have to pay a lot of money up front, it's not honest. If you have to purchase inventory, make sure there is a one-year buy-back (e.g. 90% with a 10% restocking charge). If annual "dues" are $25 or $50, that's okay but not thousands of dollars.
The best, most ethical MLM companies are invited into the Direct Selling Association. Only about 200 of the 5000+ companies in the US are invited in after a year of scrutiny, interviews with their distributors, open books and all that.
Among the DSA members, make sure that you have a product that is consumable (so people want to buy every month and you aren't constantly looking for new customers). Make sure you can bring on as many distributors below you as you want - if you can only sponsor 2 people and they sponsor 2 people, etc., that's a binary system. You will never make any money. If you can only sell through parties, it's very time consuming. It might be a good company, might not.
Consumable products include food, nutritional supplements and, to some degree, cosmetics/skin care. But they have to be unique and in demand. Luxury items, or items that only appeal to a segment of the population, are more difficult. Baskets, jewelry, kitchen gadgets, craft/scrapbooking supplies, children's toys - they may be wonderful (and I buy some myself!) but ask yourself if everyone needs them and if they need them every month. It's hard to make a living with something that people buy a lot of before Christmas and then don't buy again for months. That doesn't mean it's not an honest business - it just means it's got a difficult product to sell.
If you have to constantly recruit new customers, you cannot make much money. If you can work with customers and distributors at various levels, and if you never lose distributors who come up to your profit level (some companies call them "breakaways" because you work to recruit & train but then they ultimately can leave your downline and start their own), and if you can make money in various ways (e.g. 5 different avenues), if bonuses are available to new people and long-timers alike, then you have something.
If there is a business guarantee, free training, and the opportunity to attend training for free before you decide, you're on the right track. If you can sell in other countries, that's great. If you can have people under you that you've never met, that's good too.
Let me know if you want more guidelines.