It's easy. Ask someone else to read your page content. If they look like they want to punch you in the face (or kick you in the groin), chances are it's spammy.
REad it as another person would. ALso there are certain keywords that will scream spam to alot of people
Make the text look natural. Include the most important keyword in Title tag, Description tag, H1, H2, ingress, 1-2 times pr 100 words and include it in incoming links from internal and internal sites. Also include the work in image alt/title where it is relevant. That's a good start. Good luck!
So far I know no search engine imposed any limit for keyword spamming. Some say that 1% -2% keywords in a page is ideal more keywords may raise spam flag. But it was a backdated concept, since Google launched manual checking for search results. You better write natural & unique content, optimize in proper way, then get good result automatically. Placing too many keywords can't do anything for ur search result.