I have just started making my first ever brew and I already need some advice.

I am making a Sovereign Gold Stout from the HBS in Ballarat. Everything was going perfectly until today, when I noticed that the bubbles through the air lock have substantially slowed down...actually, almost stopped. I started the brew on Friday afternoon, so it has been going for about 48 hours now. But the bubbles should still be going strong shouldn't they?
I have a heater in the fermenter (came with the kit) and the thermometer is registering around the 24oC. Reading a few posts, I am wondering if this is too hot? Or is that ok for stout?
And if the bubbles stop, how do you know that it is something wrong or it has finished fermenting? By using the hydrometer? Mine started at 1035.
The bubbles haven't completely stopped by they are VERY VERY slow atm. In the previous 30 hours, it was bubbling along merrily.
Any advice would be greatly appreciated. And pease bear with me, I am new to this and will probably have HEAPS of questions as I go along.
And thanks to those that set this forum up....