Many studio now sadly add a watermark now so you can go on their website and look at the releases around the date the video was posted and see if you can identify it this way(assuming they post them as they come out).
If the credit section is still there, you can check for the director name or the actresses in there and then look at japanese text to match the characters so you can search for it.
You can also use the identification section in this forum to identify the actresses if all else fails.