Yeah...and no. There's no "internal striker" in the slide like a real glock. Field strip your pistol when you get it and take a close look at what touches what between the lower hammer/trigger mech and the upper slide.
See, you didn't specify what pistol in the original post...that's why I said just leave it cocked until you get more familiar with what you have. Some pistols 1911/hicapa/etc...have exposed hammers and you can do what stalker said and use your thumb to block it.
You can stick your thumb over the little paddle hammer of a glock...but take a good look at it first and try not to let it slip. You won't get much leverage on those...but it's not a freaking huge spring either.
Some pistols have decockers (226/USP) that let the hammer fall to a half cocked position...they're ok to decock with the slide off.
But really...there's rarely any need to drop the hammer when it's field stripped. But you'll be tempted to (everyone is)...and it's not uncommon to cause excessive wear and tear dropping the hammer without the slide in place.