WKT - I've hit my head a couple of times in race crashes, with broken helmets. I didn't get any noticeable concussion symptoms from those - though I did lose hearing in my left ear after the last one. (It's possible the hearing loss was caused by something else, but we're not sure.)
But, I did also smack my head on a door frame while taking my dog out - it's a long story - and that definitely gave me a concussion as I was suffering from a massive headache and nausea for two days.
At this point, I tend to lean toward better safe than sorry, but if you're not experiencing any symptoms, you may be OK.