Its gonna be hard to tell if its Brett just based on a slow growth. Another possibility is that it just doesn't like growing on your agar plates, either based on a lack of nutrients that its looking for or an incorrect nutrient. That being said, I wouldn't go tossing it out. Give it time to finish up the starter and give it a taste. If it tastes decent, consider making up a small batch (1 gal?) and pitch some in. If you're worried about it actually finishing fermenting that small batch, if you've got another sample of yeast laying around you could use that to primary ferment your small batch and then pitch your wild yeast like you would brett in a secondary. Give it plenty of time and patience, then if it tastes good build it up from there .