How can I see if my code is "concurrency safe"?

Hi there!

Today I realised that my knowledge concerning how postgres handles concurrency is not very good, and its even worse when it comes to using that knowledge in real-life.

Let me give you an example.
I have this table 

create table userpositions ( userID int,  positionID int, unique (userID,positionID));

For a given userID there can be many positionIDs.

There are then two operations performed on this table, the first is "select positionID from userpositions where userID=..." to get all positions for a user, and the second is to replace all positions for the user with new positions. For this I was thinking about running it in a transaction

begin;
  delete from userpositions where userID=...;
  insert into userpositions (userID,positionID) values ....;
commit;

But will this work? I don't want select to return empty results, I don't want two inserts running causing a unique violation.
Experimenting with it tells me yes, it will work, but how should I reason to "convinse" my self that it will work?

Quoting the docs:  "The partial transaction isolation provided by Read Committed mode is adequate for many applications, and this mode is fast and simple to use; however, it is not sufficient for all cases. Applications that do complex queries and updates might require a more rigorously consistent view of the database than Read Committed mode provides."

How do I know I'm not creating one of those complex queries? Or to put it differntly, how do you identify a complex query with potential issues? Any special techniques to analyze?

/J