It's really not that hard to tell when someone is desperate for anything, really. A lot of it comes down to overly praising, being overly nice to the point that it feels like they want something from you, and just being way too awkward for a normal social interaction. Think about it this way:

The person with the most leverage in any negotiation is the one who's more willing to walk away from the table.

A lot of the girls who are at least around average have a good handful of guys who have or are actively showing some kind of interest in them, but many of them suck in some way. Socially, it's just much more common for men to be the initiators and women to be the ones to accept or reject intimacy. Guys who have no prior luck with girls in the past, because they have fewer opportunities to interact with a girl they might be interested in due to being the ones that have to initiate most of the tine, are more likely to be desperate to make any shot they shoot work. Conversely, guys who have had success in the past understand that rejection will happen, and it's not a big deal, and they go into every approach with the mindset that there's plenty of opportunities to try again so long as they actually look for them and try. They treat it like a numbers game instead of taking each rejection personally. The first group comes off as desperate, and the second comes off as confident, and a lot of that is shown in how much they care about appeasing the individual girls they talk to.

If girls are interested in you while you're in a committed relationship, that means you ARE plenty attractive, but you just come off as too desperate when you're trying to get with them. And luckily, that's very fixable.