うちに has two rather confusingly different meanings.

One is in line with its never seen kanji form. Whilst. Do x whilst y.

Another means do x before y's situation changes.

The two are quite different as to when x should be done. Is there a way to
spot which is meant?
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grammar
うちに spot the difference
I don't think those meanings sound very different, but if you were to give
some examples we would be better able to address your question.

～うちに can be used in a number of (grammatical) situations, and in almost all of
those it can be understood in English1 as "while".

> 考えているうちに分からなくなった。  
>  _Kangaete iru uchi ni wakaranaku natta._  
>  (While thinking about it, I got lost.)

However, **when ～うちに follows a negative** it can be understood in English as
"before".

> 雨が降らないうちにテニスをしてきます。  
>  _Ame ga furanai uchi ni tenisu o shite kimasu._  
>  (I'll go and play tennis (and come back) before it rains.)

Additionally, when ～うちに is preceded by something for which the beginning and
ending is certain, ～間【あいだ】に can be substituted.

> 子供が学校に行っているうちに／間に本を読みます。  
>  _Kodomo ga gakkou ni itte iru uchi ni / aida ni hon o yomimasu._  
>  (I read books while my child is away at school.)

I hope this answers your question. If not, I am more than willing to expand
it.

* * *

Notes:  
1 Though as a matter of personal opinion, I don't believe this is the best way
to understand things in a foreign language.

* * *

References:  
Makino, Seiichi and Tsutsui, Michio. "A Dictionary of Basic Japanese Grammar".
Japan Times, 1994.
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For example,

> Do x whilst y  
>  [明]{あか}るい **うちに** (≒明るい[間]{あいだ}に)[帰]{かえ}ってきなさい。(Come back while it's
> bright)  
>  [温]{あたた}かい **うちに** (≒温かい間に)食べなさい。(Eat while it's warm)  
>  子どもが[寝]{ね}ている **うちに** (≒寝ている間に)[洗濯]{せんたく}します。(I do the laundry while the
> child is sleeping)

can be rephrased:

> Do x before ys situation changes  
>  [暗]{くら}くなら **ないうちに** (≒暗くなる[前]{まえ}に)帰ってきなさい。(Come back before it gets dark)  
>  [冷]{さ}め **ないうちに** (≒冷める前に)食べなさい。(Eat before it gets cold)  
>  子どもが[起]{お}き **ないうちに** (≒起きる前に)洗濯します。(I do the laundry before the child
> wakes up)

(Use a negative form for the latter format.)
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