A pure scalp is something like +120/-115 with a guaranteed profit. You can also get pure scalps on 3 ways +200/+200/+205. (Obviously simplified for this post) You can use a calculator available at bookiebusters (I am sure there are other places) or write an excel spreadsheet to analyze opportunities. Some good sites to use if you really want to scalp are e-sportstats.com (About $1200/year) or betbrain (Free - but more non US concentrated)
For pure scalping, the best sports are Baseball, tennis, obscure sports, soccer.
Now non pure scalps would include:
Two games of -110 when you are getting a 20% bonus on both sides.
Also some people would consider something like -110 ML/ -110 +2 as a scalp. To figure these out you need to know the value of each point in each sport.
The easiest way to scalp is to use a program like live lines that finds them for you.
Good luck.
Sean