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Decide what you are actually asking
There are two different questions here: "is this person safe to meet" and "is this person being unfaithful". They need different evidence and lead to different conversations, and mixing them produces a search that satisfies neither.
Write down which one you are asking before you start. It will keep you from treating every ambiguous hit as confirmation of the worse answer.
What a username check can and cannot show
A correctly built check can show whether a handle resolves to a public profile on a platform that publishes profiles. That is a fact you can verify by opening the page.
What it cannot show is whether an account is active, current, or connected to the person you know. Old accounts persist for years after people stop using them, and deleted accounts leave almost no public trace at all.
Reading results without catastrophising
Sort your findings into what you saw yourself, what a tool claimed, and what you assumed. In most cases the middle and last columns collapse once you actually open the page.
Pay particular attention to dates. A profile created before your relationship is a different situation from one created last month, and that distinction is only visible if you record what each page showed.
The conversation you owe yourself
If what you found is genuinely conclusive, the useful next step is a direct conversation, not more searching. If it is not conclusive, saying so plainly is fairer than presenting suspicion as evidence.
And if searching has become a loop you repeat weekly without new information, that pattern is itself the answer to a question no scan can resolve.