Better still, wise up. Don't take anything anyone online says at face value. If it really matters to you that someone is from Finland, get this confirmed some other way. Or don't let yourself get into such a state that when it turns out they're actually Egyptian it makes any difference to you. Assume everyone you meet online is a 45-year old paedophile until you've actually met them (in a safe location) face to face.
But to answer your question, basically no, not with 100% certainty. Even if you can get their IP from Yahoo you can do a reverse DNS lookup (Google it) on their IP address and that might give you some idea, but there's no guarantee that if they're using a Finnish ISP they're actually from Finland; they might be using a proxy server, and if it's an anonymising proxy then you've no idea where they're from, although if you see that they are using an anonymising proxy this might clue you up that they're trying to hide something (which isn't necessarily a bad thing - they may just be paranoid). But it's far better in my opinion not to let "I'm from Finland" make any material difference to your life.