My bf has online dating profile
To search for their profile picture, save their social media profile pictures (and any other likely candidates for dating profile pictures) and drag them one by one into the google images search bar.
To search these sites, you may need to alter your profile to match your sigifigant other's potential search preferences.
If all else fails, you can hire a professional investigator to find out for certain whether your signifigant other is using a dating site.
The first step is to be aware of common dating sites and apps (many of which are intended to look inconspicuous) so that if you stumble upon them on a shared devices's apps or browsing history, you’ll recognize them.
Unfortunately, dating sites and apps allow people to have secret interactions and flirtations and, in some cases, entire relationships without the risk of their partners catching them in the act.
Fortunately, these sites aren't necessarily as anonymous as would be adulterers and cheaters would like to believe.