I bought myself an iPad last night and was told it was on tmobile was wondering is it possible that it's locked to that network or open for any sim to work in it ?
Is there anyway to find out without buying sims from different providers?

Whyt not ring up T Mobile and see if they will unlock it for you? You will just need a PUK code or something simlair cant remember the name. They may charge you for the priviledge

if its t-mobile it will work on orange too

If you go to the O2 shop they will give you a sim card for nothing and you dont have to sign up for anything either
Its also the best one if you dont go online using 3G much as you only pay by the day rather than a rolling contract!

I did think of contacting tmobile but I'm not 100% that's who it's with I have put my O2 sim in from my iPhone and it says invalid sim and I hate orange after there customer service I've had in the past :-(

Get a wireless hotspot that takes the sim...

http://www.amazon.co.uk/Unlocked-Wireless-Modem-Huawei-E585/dp/B003VHT37U/ref=tag_stp_s2_edpp_url

Use any of your devices through wifi. One option.

Also, if it's a network lock then to the best of my knowledge Orange and T-Mobile are not 'the same'.

i.e. if it was locked to o2 then it wouldn't work on Voda just because they use they same frequency.