keyword stuffing is very easy to detect, its similar that if I start talking about you using your name instead of he, its call stuffing now if your name is keyword which ill be used again and again then its keyword stuffing so simple :-)
Using a target keyword on the targeted page too many times or too much is keyword stuffing. Suppose you have to target a keyword "SEO" on a page, now you have decided to write something to put on the page like this:
SEO is a process of making a website search engine friendly. SEO can boost your website ranking. SEO is cheaper than the adwords and SEO can be done by simply reading the SEO guides and SEO books available free online. SEO can make your business more profitable and SEO is also suitable for even small business.
Now what to note here is the use of word SEO; Where we can use It instead of using SEO. The attempt to use the targeted keywords many a times leads to the keyword stuffing.
Also if you have used the same SEO word in Alt tags on every images present on the page instead of using the relevant Alt tag what the image is all about increase the number of keywords. But many a times it leads to problem rather than helping you in gaining the good ranking for the targeted term.
A content must be written for the users and not for the search engine bot in order to overcome the keyword stuffing. All the Alt tags should be chosen keeping in mind the image and not the keyword.