If she texted you immediately after getting your number, it's bound to be her. You want to keep the chill/cool/sexy vibe on a site like Feeld. It's like the "secret society" thing so don't mention the site, and don't request confirmation. That being said, girls who text me but don't identify themselves usually tend to be difficult in my experience. Even if you just gave her your number, there's no point in making you guess. It's an indicator of potential compliance issues.
Think of it this way - if she texts you immediately, who else would it be? And obviously you want to appear confident, so don't be seeking confirmation it's her unless she starts acting really weird and incongruent.