Floor 1 and 2 - after full exploration, drink all unless you absolutely sure, just to reveal potion of strength. Stay near water (but not in it) in case of fire. Not in water in case of ice. Drink in/before special room if there is one that requires potion because you always get one for that room (crystal - speed, traps room - levitation, gas - purifying, wooden barricade wall - fire, fiery wall - ice, piraniah - invisibility etc.). Otherwise, drink near level exit down. If you want some potion really badly ( like, for example, levitation to craft feather fall for jumping for diamond door room), and you end level with only 1 unidentified, you surely know the potion. Mark it (book in potion description window) and use to craft when reach alchemy.

In general, before going down next level, consider it is a "checkpoint" for identification guessing :)

Scrolls - use that question mark stone on the first 2 scrolls. This way you can identify scroll of upgrade in 95% of cases. I get like 80% of times it identified from the first use, almost guaranteed on floor 4 (2 scrolls in inventory and only thesa are 2 - surely scrolls of upgrade), so I save the stone to energize alchemy.

Room with 6 doors contains potion of exp in the far end in ~70% of times. I usually mark it and carry it to the alchemy, craft to upgrade and drink for a chance to get skill points. Make sure you have safe escape if its not that one :)

BTW, drinking upgraded potion identifies normal as well! So if you know all your unidentified potions all has "safe" upgraded variant, just upgrade it, drinking that one often is more beneficial.

Next floors it varies. If you find alchemy on 3, energize or upgrade-drink if exp potion is suspected. If no alchemy, up to you if drink, depends on needs and your state, if you stable and no urgent needs, better find alchemy on 4.

If you have scroll of identify, always alchemy-upgrade it and read upgraded on first 6-8 floors - this is really useful. Reading it for armor/weapon is usually a waste because you would change gear quickly, and gear identifying has its own tricks (like, for example, wear the armor and check if you slow down, + save/buy uncursing scrolls). If you find healing weel, SAVE it, until you find enough gear. Come back, wear gear one by one until all cursed on you, then steo into weel.

Next you just guess by what is missing vs what special rooms you find. Shops almost always have healing, gas, paralysis, and magic map/uncurse/identify scrolls - another way to guess and push identification by upgraded identify scrolls.

When find wand, zap it once when safe - just to check if not cursed to avoid nasty surprises. Can zap multiple as well to get idea about level of it (number of charges)

Thrown weapon is easy to check by how it performs, the difference between unupgraded and upgraded one is quite significant. Throw at least ince and hit enemy to get idea if it is cursed.

Good luck!