- In general: Shift+D is your friend. You can determine any time if it's worth trying out a potion or scroll.
- potions: up to maybe level 4 I directly drink them all. At least until I have found the strength- and life-potions. Even that potion of incineration will not kill you at full health. And on the first levels you usually can dare trying. You can also reduce risk by trying to drink an unknown potion nearby water in order to distinguish the flames. But not too close or you will be damaged by steam caused by the potion-flames in the water. You can also go a level up or nearby stairs leading down in order to minimize the problems of drinking a potion of descent.
Once I found the most important benevolent potions, I start throwing unkown potions at enemies. That will not help identifying some of the potions, but is always fun if you find out that it has been a potion of incineration, paralyze, descent, confusion. But once i found incineration I try out the others and take the disadvantage of the others. Depends on the level I am at. You can also wait testing potions until the start of a level 3 or so (or one with a treasure room) because if you drink a potion of detect magic you will have roughly identified all items in your backpack as well. hm now that I write it...I think I have to start testing potions later than I did so far.
- scrolls: In general they are less dangerous to test. The biggest danger is on more advanced levels, when reading aggrevate, summon or cause fear if you have an ally. The bigger loss is to waste a scroll of enchant or identify. That's why I do not use them directly, but wait until I got at least one item that I REALLY want to identify or enchant.
- armor and weapons: Some armors and weapon do not have runics at all. Like platearmor. You can partially identify by carrying them after sorting out cursed once with a potion of detect magic.
- staffs: can be tested on monsters most of the time.
- wands: should not be tested that way, but the wikie mentions something about testing them on caged creatures:
http://brogue.wikia.com/wiki/Category:Wand
- rings: try to roughly identify them with a potion of detect magic, then wear the good ones.
- items of a treasure-room: You can not determine if the are enchanted or cursed, but you can pick them up to get a more precise description. If it's not good at all, then drop it again and take something from a different altar.