If they are online at odd hours, you should ask them why.
Is being on an online dating site cheating how to get ready for dating
This is obviously because they don't want their partners to know they are talking to an online date.
Very often, married men are online late at night while their wives are asleep.
If you suspect your signifigant other might be using a dating site or app, there are a few ways to confirm your suspicions, including through starting your own infidelity investigation.
You'll need to check their phone for common dating apps, conduct a Google search using personal details like their email address and, if necesary, create test accounts on popular apps and sites to see if your signifigant other is returned as a potential match.
Many sites and apps (like Tinder) only allow people to see each other based on location and mutually matching search criteria (like age and sex).