I think he's happy. It's hard to tell.

我想他是幸福的。这很难说。

《牛津词典》

It's hard to tell what effect this latest move will have.

很难说最近的这次行动将会有什么样的影响。

《柯林斯英汉双解大词典》

The immediate future is clear, but it's hard to tell what lies beyond.

最近的将来已经明朗，但更往后就很难说了。

《牛津词典》

It's hard to tell.

难说。

关于巴西 - SpeakingMax英语口语达人

For a couple of reasons. In some ways, this would be nicer, do expected cases, it's going to tell you on average how much you expect to take, but it tends to be hard to compute, because to compute that, you have to know a distribution on input.

关注最快的情况，在某种意义上来说，因为一些原因这样想挺不错的，当我们处理一个给定的问题,计算平均时间的时候,是很难计算的，因为你并不知道输入的分布情况,这些输入会是怎么样的呢？

麻省理工公开课 - 计算机科学及编程导论课程节选

Well, it's a little bit hard to tell exactly, isn't it?

这个,有一点难以分辨,是不是

耶鲁公开课 - 聆听音乐课程节选

But it's hard to tell exactly where society comes down on the balancing the risk-benefit scale.

FORBES: Safety Vs. Efficacy

"It's hard to tell if TV or the Internet was the driver, " he says.

FORBES: Magazine Article

"It's hard to tell anything on the first date -- you're so on the surface, " she explained.

CNN: Why traditional dating is dead