If all else fails, you can engage professional investigators to find out for certain whether your signifigant other is using a dating site.
Icaught my boyfriend on a dating site
To search for their profile picture, savetheir 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.
Icaught my bf on a dating site
Sites and apps (like tinder) only allow people to see each other based on location and mutually matching search criteria (like age and sex).
To search these sites, you may need to alter your profile to match your sigifigant other's potential search preferences.
In that case, one option is to create your own account on one of these sites and then search for your husband or wifes physical attributes and location to see if you find a matching profile.
Fortunately, these sites aren't as safe and anonymous as would be adulterers and cheaters would like to believe.
Though my trust in him now was shaky, we went on dating for a few weeks as i hatched a plan.