We generally want to give those we care for the benefit of the doubt, but there are instances where we can't really afford to do that. As just one example of why we need to be vigilant: there are a lot of scammers and con-artists out there, and while the online dating scene has changed and it has come to be a lot safer than it once was, the fact remains that it's still a popular avenue for scammers to take to target new victims.
We don't always want to do the research because we're scared of what we might find out, but knowing for certain, one way or the other, beats worrying about it. Take these signs as red flags and conduct a background check or other research:
You've Never Met Their Friends
On the first three or four dates, it makes sense that you wouldn't really want to introduce your new boyfriend or girlfriend to your circle. If it doesn't work out, then you have a lot of explaining to do for nothing. After a couple months of dating, on the other hand, you start to wonder why they don't want you to meet their friends.
They Give Very Specific Instructions
They want you to call them after five but before eight, never on weekends, Wednesdays, or Fridays, only at their work number, and never on holidays. There's nothing wrong with setting boundaries, but highly specific instructions for when, how and where you're allowed to talk to them tends to come with the territory of dating a married person.
If you simply cannot get a straight answer out of your partner about where they live or work, previous relationships, family or friends, then they're probably dodging your questions for a reason.
A boyfriend or girlfriend who isn't forthcoming or is secretive might not always be a scammer or a criminal or married, sometimes they just have trouble opening up to new people, but it's better safe than sorry, and it's better to do the research early in the relationship rather than cause a rift later on.