While not quite the same as Read Receipt, you can hover over a profile and it will tell you when they were last online. Given the purpose of the site, someone logging in and NOT reading their messages would seem "off". At a minimum, it does lean towards them not being very responsive.
It's likely that "No answer" is your answer. Not always though, as some couples both need to see and discuss any message before responding, but if it's been a week and they've been on every day since, it's a fairly safe bet you'll get no answer. Even the Read Receipt doesn't give you the full answer, however someone that is at least interested in you is less likely to leave you hanging like that.
If this happens to me, I'll usually tag the profile with a note saying, "Sent message on xx/xx/xx. " when I sent the message and after a while will add "Got no answer" to that note. Lets me know not to bother trying again.