How do I know what type of soul a creature has?

At first, this seems pretty straightforward:

• Petty gems hold non-humanoid mobs, below level 4
• Lesser gems hold non-humanoid mobs, below level 16
• Common gems hold non-humanoid mobs, below level 28
• Greater gems hold non-humanoid mobs, below level 38
• Grand gems hold any non-humanoid mobs, regardless of level
• Black soul gems hold any mob—humanoid or non-humanoid—regardless of level

So if you want to use your grand soul gems efficiently, you'll want to reserve them for mobs level 39 or higher.

But determining the mob's level is the tricky part. You can use the console command getlevel.

However, you don't have to worry too much about that, especially for soul gems. Bethesda knew you wouldn't know what level mobs are, so Soul Trap will automatically use the smallest possible gem you have available.

So if you want to efficiently use your grand soul gems, always have a stockpile of the smaller gems handy.

Determining monster soul types

Enemy souls are determined by their level, but without the console or an encyclopedic knowledge of enemy statistics, this information is not particularly useful.

Here's a quick chart of the most common types (or at least the most common types of enemy I tended to encounter on my playthrough), but there's a full listing over on the UESP should you be curious about a particular type of enemy:

• Petty - Most small farm/non-hostile animals (rabbits, chickens, cows, elk, fox, deer) and mudcrabs.
• Lesser - Restless Draugr, Sabre Cat, Bear, Troll, Falmer and Falmer Skulker
• Common - Snow Bear, Cave Bear, Frost Atronach, Frost Troll, Draugr Scourge
• Greater - Giant, Storm Atronach, Low-level Draugr "boss types" (ie, Deathlord at low levels)
• Grand - Mammoth, High-level Dragur "boss types" (ie, Dragon Priest)

Some creatures' valid level ranges vary depending on character level, and therefore their soul size changes depending on what your level is. I have tried to stick to mostly monsters who will consistently have the same size soul in the listing above.

Dwarven constructs (spiders, spheres, etc) have no souls or cannot be soul trapped. However, they sometimes carry soul gems, and these are occasionally already full. Summoned creatures can't be soul trapped, which is a change from previous Elder Scrolls games.

Humanoid souls and black soul gems

Black soul gems are an exception. They can hold humanoid souls, which are considered to be "grand" in the classification system above - regardless of the humanoid's level. They will also take on any level of monster souls, if you soul trap a monster and no other suitably-sized gem is available. "Humanoids" include the following:

• Any NPC from the 10 playable races (ie, Argonian, Khajiit, Nord, Imperial, etc) such as people living in town, or bandits/other hostile NPCs you find in the wilderness
• Humanoid Dremora (fairly rare in this game)
• Vampires

Note that Dragur, Falmer, Werewolves, and Giants all look humanoid but are not considered as such by the game. The quick rule of thumb is - does it speak my language? If so, it's likely to be a humanoid. As far as the law is concerned, if you could murder someone without it being a crime, there's no additional penalty for soul trapping them while you do so.

Soul Gem Do-over

Sometimes if you screw up and trap, say a "petty" monster soul when you only have grand soul gems available, the smaller soul will take up a large gem. If you end up with the wrong size soul in a gem, typically you can drop it on the ground and pick it back up to clear it. Whether this is a bug that will eventually be fixed, or a feature is unclear.

Soul strategy

An interest in trapping souls means that you're likely into enchanting. This means that you're going to need a supply of low-end souls for skill increases, and a supply of high-end souls for creating awesome gear.

If you're farming low-quality souls to enchant junk items for skill increases, I'd follow a river and soul-trap mudcrabs - they're plentiful and easy to kill. Buying small soul gems (common or smaller) from court wizards ensures you'll always have a supply.

For farming grand-level souls, mammoths are your best bet, although plan on killing Giants (which have greater - not grand souls) at the same time.

There are a few high concentrations of "leveled" souls in the game, which can speed up your collection of grand-level souls. If you're level 39 or higher during the quest The Break of Dawn, all the Corrupted Shades in the temple will have Grand Souls. The magic anomalies you encounter during the Mage's Guild questline also carry soul gems and have leveled souls for trapping - being level 23 should cause them to create grand souls.

You can lookup information on this page of a Wiki. The article contains a list of soul gems with the corresponding sample of creature that match this size.

Plus, I'm must check on the game, but I remember that using the gem soul to recharge a weapon does not consume (readings seems to say it does?? I have so many that I'm not sure.) the gem, so you can reuse it. Instead, creating a enchanted weapon uses the gem in the process, because you need to have a container in the weapon to store energy.