That is the key to success in being a freelancer. You have to read the client. And read him quickly and quickly recognize those who are really interested to pursue some idea or those who only waste your time without willing to pay for it.
So what I do: I scan the client during our initial conversation, chat or audio. If he is in rush, does not have time to type or explain, he does not seem interested in pursuing his idea. For those, I go straight in the head telling them that research will take X hours and it will cost Y dollars. As a result, he will get a paper on the research. You can guess, 99% such clients go to another contractor.
For other, I walk on a thin ice. Ask them to describe what they need, and then tell them it will take for example 5 hours for the research. After that I either ask them if he has budget to finance research. I explain to them that I can do a quick research up to 1h, but results would be such that we cannot rely on them entirely. Or if he wants a more quality research, than we can set an hourly arrangement. You see, I always give THEM option to choose which way they will pursue.
Now, if the research will result in your estimation, then so far I have not been able to find a way to be paid for estimation. It happens that estimations that take up to 1h result in job, and those that I do over the weekend and take over 3h usually end up with client not getting back to me again.
NOTE: when I say estimation, I do not refer to detailed estimations. NEVER EVER give detailed estimation for unpaid work. There are many cheap hunters who need estimation and are not willing to pay for it. I refer to 1-2 page excel estimation.
PS. lastly, it also depends how desperate for work you are. if you need work, then obviously you will have to do everything to get the job.