If I am interpreting this correctly, strictly speaking you never can find a limit by "testing it out" (which I interpret to mean writing out "enough" terms to determine by inspection what it converges to). It is quite possible that the sequence is .99, .999, .9999 (1- .1^(n+1) is the general term) for the first, say, million terms then changes to some other rule entirely. You must be able to determine what the general term is.]
(Problems where you are given a finite sequence of numbers and expected to determine the general term are common in mathematics- the ability to recognise a pattern is an important skill in mathematics- but, strictly speaking, they should state that the pattern does not change.)
Assuming that the pattern in .99, .999, .9999, .99999, etc. continues then you can see that it is approaching 1 by 'looking at it', but if you want to prove that, you should rewrite it as 1- .1^{n+1}. Whether or not you can recognise a limit "by inspection" depends on how familiar you are with sequences. It is proving that limit that typically requires writing the general term in some specific form.
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