Both forms "спросить у кого-то" and "спросить кого-то" are equally valid - the difference is very, very subtle and you can find a lot of answers in internet that are nothing but wrong. It wouldn't be oversimplification to say - those form a virtually equivalent.
The only difference I'm aware of - and it's only a statistical observation, I can not provide any grammar rule or other evidence - when it comes to inanimate objects, the "у"-form is more likely.
For instance, there's a famous song which contains following lyrics: "Я спросил у ясеня, где моя любимая". One can not say "я спросил ясень" but "у ясеня" sounds more natural. I'm leaving behind the brackets here the fact that it's strange to communicate with inanimate objects but I can imagine following phrases:
- "Я спросил у тополя" (and less likely "я спросил тополь").
- "Я спросил у талисмана" (but less likely "талисман").