If you "say the right things" to the doctor then s/he will refer you to a paediatrician. You want a developmental paediatrician but you'll just as likely get a general paediatrician.
"right things" are essentially conveying to the doctor that there are things your child doesn't seem to be able to "get" or process: it might be imaginative play; it might be reading other people's body language; it might be that he can talk well but seems to struggle to understand what others say to him; it might be problems with eye contact.
Emphasising naughty behaviour, on the other hand, isn't such a clear signal for doc. You can mention it , but don't lead with it. Ditto unusual or precocious interests in numbers/letters etc. Very relevant, but don't make those the first things you say.
You'll meet with the. paed. and go over the history.
Believe it or not, aspergers is being "abolished" over the next few years from the US diagnostic manual that the UK also follows,(your GP may not know this) so be warned that you might end up with a diagnosis of ASD.
Re the labelling, come over on to the special needs board to talk in more detail. But as soon as you have the first appointment, even the GP's appointment, ask what the process is. In my area, they were very helpful and clear. Paed. recommended multi-disciplinary assessment and told me the first step was me giving my consent. So I knew exactly at what point I'd be putting the matter in someone else's hands, as it were. Others have gone to a first paed. appointment and got a dx there and then, sometimes before they were ready to take that step. But the largest group are those who have fought long and hard for a dx. If your child has good speech, that can "mask" the processing/sensory problems, so it can be harder to get the problems recognised.
Someone on the SN board recently said she'd found it 95% helpful 5% unhelpful to get a label for herself. For our son, we felt it would be 10% helpful, 90% unhelpful - it's different for every child, every family, and in every year of their childhood. The main problem with dx is the generalisations that people can make, but you don't have to tell everyone about it if you do go for it.