Most transsexuals can be divided roughly into three categories regarding passing. (1) Those who pass very well in their identified gender. These tend to be people whose height, body-shape, and general appearance and manners make it easy for them to be accepted as male or female even when not wearing gender specific clothes. These people tend to be relatively rare in pre HRT mtf transsexuals. FTM's are in my opinion much more passable. These people are the lucky ones. Only the voice might give them away or an accidental slip into stereotypical behaviour of their biological sex will make people perhaps question their gender.
(2) Transsexuals in this second group tend to pass well in busy places, like malls, shopping centres. Places where there is a lot of activity, people coming and going, and nobody is paying too much attention to the people around them. Where the second type of transsexual is vulnerable is in of course quiet, less busy places, and places where people have the opportunity to take a good long look around themselves at the other people present, and any anything about you that doesn't seem quite right will probably be noticed and you may well hear 'oh my God is that girl a guy!' Buses, trains, cinema, theatre, supermarket queues are places that this second type should try her hardest to avoid. Hormones and of course SRS will make a tremendous difference to the passibility of transsexuals who fall into this second category. However, until you have been on hormones 6 -12 + months, you will have to try to avoid the above situations and try and work on downplaying those things about you that undermine your passibility. Things like clothing, maybe you are dressing too feminine or not feminine enough? Do you wear clothes that inadvertently emphasise your height, making people think ' that's a tall girl, (looks closer)notices large hands, and big feet... oh my God it's a ->-bleeped-<-!' ? Perhaps you are overdoing feminine gestures and mannerisms and drawing attention to yourself? It is difficult to get body language etc right, too much will give you away more than too little though. Hair is said to be an excellent indicator of gender, type into a search engine 'feminine males' and you can see at a certain website just how well long hair makes a genetic male appear feminine. While having long(ish) hair will go a long way, and may well be enough in a number of mtf transsexuals, others will have to review their clothing and/or mannerisms if they are getting 'read' too often. Really working on your female voice will help people in this category, because if you are called out and you can speak in an acceptable female voice then hopefully your accuser will say 'Oh my God I am so sorry!'
(3) The third category is the one that I fall into. I am very tall, I am taller than 95% of the genetic males that I pass each day. I couldn't pass in a coal cellar at midnight. Body language, mannerisms, gestures, clothing, the company of sympathetic genetic females, long hair, breasts, a very good female voice(see CandiFLA - on YouTube) all these will be to no avail. Height trumps all of the afore-mentioned together.