I've had a straight thru pipe fitted and i think it might be blowing, as it sounds this way, how can i check for sure?

cheers

if you can cover the end completely, do it. within a few seconds, the pressure should build up and the engine should labour. if it doesn't, and you hear a hissy whizzy noise, you've probably got a leak.
if you can't cover the end, jack her up and take a peek. around any blow on an exaust, you will usually find sooty deposits (black stuff, not the puppet lol).

cheers, i was told not to suffocate the zaust like that tho, as it can blow up the engine (outlet valves?)

i only mean for a few seconds, with your hand and the car at idle speed.
you should be able to tell immediately, although if this worries you, check the pipe itself for sooty bits?
i had a universal backbox on my last car, that always sounded like it was blowing, but it never was, just took me a while to get used to...
could just be that...

Hull ba is right, thats the way to do it, alternatively, jack the car up, go under it, [b]but not without axle stands[/b] with the engine idling, move your hand along, close to, but obviously not touching the exhaust pipe, if there are any leaks you will feel the hot gasses escaping from wherever the leak is.
Pay particular attention to pipe and box joints.