There are web API services your app could contact to obtain the rough location of the ip address. It's not at all guaranteed to be the correct country of the device, as your user may be using an out-of-country VPN service, but it's probably the only thing you can do without asking the device for location.
Here's an example service. I know nothing about the quality of this service (I picked this one at random for this answer). There seem to be many out there, so do your homework as to which would be best for your users and yourself.
There's another approach you could take: simply ask the user, and have faith that most of them will answer honestly. If their answer doesn't make sense per the timezone of the device, maybe suspect their answer is false.