Question : Problem: how can i verify if someone is using my wireless router furtively?

how can i verify if someone is using my wireless router furtively?
i've a dlink but it's a general question.
what program can i use?

Answer : Problem: how can i verify if someone is using my wireless router furtively?

Then I guessed right for the version you have... versions of the DSL-G624T made after July 2007 don't have a Home tab along the top of their web interface, so that would've been very confusing.  :-)

On the Home tab, under the WAN button, the Clone MAC Address button only appears with Dynamic IP selected for the WAN Setting. If that's not what you're using, you might rethink wanting to change it.

If you do still want to change the MAC, and if the WAN Setting is currently PPPoE/PPPoA you should write down the username, password, connection type, et cetera before you change the WAN Setting to Dynamic IP, in case you need to re-enter those credentials and settings afterwards (everything but the username, password and connection type are probably still on their default settings, but record them anyway).

You can make it any MAC address you want; usually the MAC of the ethernet card in your PC would be used, not that of the wireless card. Just type it in the field above the button like 00:16:CF:85:43:21 (that's a Broadcom adapter's MAC address, btw). Then click the Clone MAC Address button, click the Apply button, then you'll need to save it on the Tools tab, System screen... click the Save and Reboot. When it comes back up you'll need to go to the Home->WAN screen, select the WAN Setting you had previously, possibly enter the saved credentials again, click Apply, then Save and Reboot AGAIN on the Tools->System screen.

Did you get WPA encryption working, btw?  Leaving security disabled is like having the door of your home unlocked all the time. Sure, many do that; unfortunately, there are people that might take (bad) advantage of that situation if they happen across it, too.