Theoretically, this should be easy - follow the trail of 'terror' and 'terrifying' fear-mongering!
According to merriam-webster.com:
'Terror' is :- a state of intense fear
'Terrorism' is :- the systematic use of terror especially as a means of coercion.
Thus we can identify both small groups, as well as large groups and organisations fitting this definition. These are the little guys - the smaller and regional groups, and there are the big guys - nation states, multi-national companies. Both engage in 'terrorist activities' but there is vast difference of scale. There is often interaction and co-operation between the big and the little guy, with states overtly or covertly funding groups that are labelled 'terrorist'. Terror may be direct or by proxy, and it may also be a sustained campaign to instill fear and 'terror' in the hearts and minds of a population, such that they may be easier controlled and manipulated.
The big terrorists would also be inclined to label the little terrorist as the only proponent of 'terrorist', and 'little' terrorism as the only kind of terrorism, for this then conceals the terrorism of the big guy by focusing attention on the little guy. Indeed, it is likely to be in the interests of the big guy to label 'anyone who opposes their terror' a terrorist. And if the big guys also control the media presentation of the narrative to the world, this puts them in a powerfully concealed position of being able to dispel terror, and simultaneously claim it to be something else (like supporting 'freedom' or 'independence').
Thus, to answer the question 'how can we easily spot a terrorist', let us first consider at a theoretical level who the different groups of 'terrorists' might be, and then let us decide whether we look at the little guy only, or whether we consider the issue in the context of the existence of the BIG terrorist too! Yes we can spot them, we can see them in action in their collective distribution of misery and suffering to those who oppose them or stand in their way.
To spell it out in ABC. We can look at Hamas, IRA, and modern 'terrorist' organisations and ignore the fact that there are simultaneously a number of powerful countries and organisations in the world using terror in a systematic way as a means of coercion.