There's a wrong way and a right way to go about ensuring not getting used by thai girls. The wrong, but sure way would be to simply never pay anything of hers. This will lead to you not being able to get a lot of girls that you would otherwise get. In this culture you really upset them with that kind of attitude.
Bargirls are extremely good at spending the maximum amount of your money at restaurants and bars. Have sex with them if you want but don't date them unless you don't care about getting used!
The right way is to pay her expenses on a date, but only if it's a real date and not just a meet up with no expectations. If you see she spends money during your dinner or party more than she would if it were her own money, you can turn cold on her and/or ask will we share the bill? If a girl is clearly splashing your money then when the bill comes and she doesn't want to pay I would still pay my own bill only, thus making it a bit of a fight but it has never gone down to that and I've been on so many dates I can't possibly count them.
Let's say you meet a girl from online but don't know her or don't think she wants to potentially become your BF, then you share the bill with her. That is, pay only your own expenses. If there's chemistry and you see it going somewhere, then even if you didn't know her from before I recommend paying the bill or at least offering to pay. But if she takes out her purse to pay her share then accept that. Furthermore if you're going to pay her share, then where you take her is pretty much up to you. Preferably somewhere where you'd eat anyway if you're by yourself.
Some friend girls will try to make you pay their share too especially outside bangkok, but if they are truly just friends you don't need to pay their share. They probably expect or hope to be more than friends with you in the future and thus some of them may turn cold once they realize you're not going to show them a good time for free.
So far I have never given a Thai girl money apart from a 100 baht or such for taxi (and even that only a few times in a year and half.) Unless it was a clearly stated price for a service (sex, or whatever), and for sure you don't have to either unless she lives with you without a job. I have however borrowed a friend girl a significant amount of money and she paid it back.
Try not to think too much about how much you spend on the girls though as it is a turnoff to these girls, but no one wants to be used so use your common sense and judgement about what you want to accept and what you don't. There's plenty of girls in Bangkok that don't ask for your money and are happy to share the bill.