Have one fitted at the moment, how can I tell what the boost is currently set to?

I'm thinking of taking it off, looks like I just need to remove the T connector tubing? It has a manual dial on it, doesn't seem to have any kind of indicator, just + and - signs...

Car is running fine but I don't want the engine damaging, the lad I bought it off doesn't know much about them but I suspect he may have altered it for higher boost but it may not be safe?

 

Fit a boost gauge.