How to determine if a word occurs more in written or spoken language?
Maybe this is an inappropriate answer.

Dictionary for I know, doesn't make such distinction extensively (I remember few words did have a mark indicating its formality). 

The reason is simple. Unlike Japanese the language itself has a clear oral and written distinction, with quite different grammar, it's definitely fine to write something the same as you say it.

It doesn't mean there isn't any difference in wording. The words you put to use reflect your attitude, education and cultivation, etc.

The features of usual spoken sentences  are:

Not concise, run-on, using common plain words for verbs, adjectives and adverbs, with many 的/了/多/着/... grammar words, simple sentence syntax.

But the level of written language is of a wide range.

So called oral/spoken language in Chinese, I think, are unsophisticated and unprocessed expressions that first come into your mind when you are about to utter words. Many well educated people, they actually say written language (in others' eyes), and on the contrary, many poor writings are close to internet chatting. (The least requirement of a written form is that you do not use many filler words and 的/了/多/着...)

e.g. Translation of the above.

spoken:

这可能算不上是答案。

我是不知道哪本词典条条目目都给分书不书面的（但有的词还是有个正式程度的标）。

道理很简单呐。不像日语口语书面语分得很开，语法都不一样，中文你照说的写，完全没问题。

但这不是说用词一点区别都没有，你用什么词能看得出你这人的态度啊，学识啊，修养之类的。

口语一般有这些特点：

啰里吧嗦，一路逗号，大白话，的啊了啊多啊着啊一堆，句子成分也简单。

但书面语的程度分别就大了。

我觉得中文所谓的口语啊，就是要说话了第一个想到的话，用的都是没来得及加工的简单东西。很多有文化的人，出口就是一般人想半天写的东西，反过来说，有的人写的乱七八糟的网上聊天似的。

written:

这解答也许难称合适。

就我所知，还无词典对词条书面程度广加类分，衹偶见标注罢了。

但其缘故无繁。不若和语言文迥异，至于语法专用，汉语笔书白文，并无不妥。

此言并非教人无别而轻词用，遣词如何，恭倨、博浅、修鄙，皆在其中。

口语大抵特点如下：

壅冗接缀，实词浅近，行句直脱，的了多着一类，赘词太多。

而书面之层次则甚蕃。

所谓口语，依我孔见，不过口将言而句已生，直白粗砺，略无藻饰。富学之人，虽言而文于外人耳。反之，寡薄者为文之不堪，反倒若如今网聊。