Robots (Algorithmic Trading)

I've been contemplating the whole bot situation and would love to gather a clearer picture of how prevalent the activity really is and what kind of trend it's inducing.

So naturally I'm trying to build a tool/sheet/app that will analyse the course of sales throughout the day and identify questionable sales, flag them as robotic in nature and hopefully provide us with some statistics to formulate our opinions from.

My question to the HC community is how would "you" identify robotic (algorithmic) transactions based solely from a course of sales historical data (or real time)?
What assumptions would you make?

Example:

• Transactions of value less than $500
• Frequency of small transactions
• The condition attached to the transaction (XT)

Perhaps there is another way to identify such activity, any insight is most welcome and greatly appreciated.