Mining these forums and posts, I found two partially-conflicting insights.

OOH, mamasans are best qualified to assist in finding the best guy for you.

OTOH, they also have - or might have - ulterior motives (e.g. - recommending a boy who won't do what you want becsue they don't have someone who will), not the least of which is getting a tip for themselves.

How to know when to trust?

Any specific mamasans, reliable or not?
 

Answer yes.  Both are true.

Do whatever you feel comfortable with.  Personally, I don't involve mamasans in any of my decisions, but I do make a point of trying to be nice to the mamasans (with the exception of a few at Lucky Boys who I just aggressively ignore).  

I would get a guy's attention and have him sit for a drink and then find out if he's right or not right for me.  Waiters can also help get a guy's attention to sit with you, but I've usually found the guys very attentive and looking to me for a signal to come down.  Repeat that process two or three times if necessary.  Occasionally I suppose it's expensive because it might involve buying a few extra drinks and little thank-you-for-sitting-with-me tips, but I just don't ask a mamasan for a recommendation.  Anyway, after a while at a bar I think you sort of know who you at least want to try to talk to without any input from anyone else.  

[Personally, and this is probably just me, I find it a bit off-putting and demeaning for all concerned when a mamasan scrambles around the bar to grab different guys to parade in front of a customer.]

But to each his own, and I suppose if you are shy they can help.  Mammasans try to insinuate themselves into the process because that's their job.  No hard feelings any which way.

Having said that, they can make the lives of the guys who work at the bar better (or worse).  So, particularly if you want your guy to look good for his boss and maybe get a tad bit better treatment at a bar, it helps to cultivate a mammasan.  

I recommend Neung mamasan at Jupiter.

He is not a pushy type and helpful also still good looking. lol

I also recommend Emmy at Moonlight.

He seems influential over models and boys.

A good mamasan is worth every penny. I fondly remember Khun Phet at Super Lex Matsuda (for newer BKK visitors, this gogo bar was in the same short sub-soi as Super A), circa 2000. She kept the place in good order, spoke excellent English (and Japanese, I'm told), and knew everything about every boy -- his physical characteristics, his personality, his language skills, what he would do/not do, etc. I fear she was the last of her breed, however. There was another good one active in Soi Twilight as recently as the mid-2010s, but I've forgotten his name. He was an ex-gogo boy himself, and he too knew everything about every boy. One time I asked which of the available boys had the biggest cocks and he jotted down a list of the top 5 on a paper napkin for my consideration. Another time I was unable to find a favourite because he'd changed bars. This mamasan recognized my description of the boy, asked for 500B, went off with it, and came back with the boy in street clothes. Needless to say, I gave him a good tip for a job well done. Otherwise, I haven't found mamasans to be of much use in recent years. 

The ones dressed as women I ignore, the others I may talk to if absolutely necessary (very rare).

I tend to ignore them in friendliest possible way and certainly don't let them be involved in my decision making process, I prefer to call prospective guys myself. 

But I'm not their enemy and few times  / BBB, Screwboys , Moonlight / I find them very helpful i.e. ' you boy  not here today but if you want I call him at home to come bar" , one at BBB was instrumental in convincing boy he should not to be reluctant to go with me  because I'm good guy. 

So keep mamasans at bay with help of 50-100 baht tip and firm statement  ' I like girls better ", works for me