Is it legal to use dungeon finder (texture pack) ??

Observation is the greatest tool. As Pab10S and jacob5089 noted, there are a few obvious tells. We also have the report system and chat logs. You'd be surprised how often a criminal (in the game and in real life) will brag to their friends about their exploits.

There are also a few other tools that I'm not at liberty to discuss, but they really aren't any more valuable than the ones that have been mentioned.

I think this would be hilarious to watch someone bumbling around a ravine or lava. When I use Schematica I'm always trying to hop on blocks that don't exist or falling in holes. I imagine it must look similar to someone using xray.

I hate when I choose a random place to strip mine and somehow find diamonds 3 blocks in.

I tried using x-ray in single player with the theory that it would make me a better moderator if I actually knew something about it. I guess I wouldn't make a very good cheater, because although it did help find diamonds, I almost always had to switch back to a normal texture pack in order to actually dig toward the diamonds, and I often missed them and then had to switch back to x-ray. It was a nightmare of switching back and forth, bumping into walls, staring at walls while slowly turning, and digging into lava.

Frankly I didn't see the point. Just dig straight with an efficiency pick at level 12, about three blocks high (so blocks 13-15), then dig over 4 blocks and dig straight back the way you came. Keep doing that, and you'll find plenty of diamond ore. Use a fortune pick to actually mine the diamonds (or silk touch if you prefer the ore). As long as you're not right up against the blocks you're mining (an early mistake I made) you have plenty of time to move back if you dig into lava or other hazards. I always carry a bucket of water so I can quickly turn lava pools into obsidian (or flowing lava into cobble).

The ease of doing things the right way is one of the reasons why cheaters have never made much sense to me.

Another suggestion is that you should always replace ANY gravel you walk on with cobble or dirt. I was mining last week, and i started walking on the gravel since it hadnt fallen... yet. On the fourth block, it updated and i landed in a pool of lava -_- Lost 6 eff5, unb3 diamond picks, an unb3 silk 1 diamond pick, 2 stacks of redstone and coal ore, and half a stack of diamond ore. Taking the extra time to remove and replace gravel it definitely worth it.

Yeah, I've fallen for the old "step on gravel just as it sinks into lava" trick too.

Oh, and of course- NEVER MINE STRAIGHT DOWN

Straight up's usually a bad call, too.

Yeah :/

Straight up ?? I always mine straight up to get out of the mine. Because every time I go to mine I forget from where I came

Dig into lava or gravel, and see how long you live.

Well, you run the risk of suffocating if there's sand or gravel above the block you're digging. There's also a possibility of digging into surface lava or flowing lava. When I'm lost underground, I dig up at an angle and never directly above myself. I've died too many times doing things when I should have known better, and then losing everything I had just mined.

I mine two tunnels up at one time, so I can jump onto the other one if something falls.

That's my tactic, particularly when playing on the xbox version.
But for going down;
Although, my variation is stand on the border in between the two blocks, then mine out one block of side at a time.
Protects against falling to your death or into lava.

When I dig straight down, I dig 2 tunnels so I know if something is below one of them

This should help.
http://www.minecraftwiki.net/wiki/Tutorials/Things_not_to_do

One key "trick" someone missed is F3. Mobs will accumulate around you in the dark areas. So, when you see an entity count of xx/xx, you are likely near a large dark area. For sure it will be at least a large cave system!

I've spent the last 2/3 days doing this, trying to light up areas around a grinder. And that's on top of the time I've spent explroing the caves initially!

Am I the only one that, after days and days of hard mining without live map, F3 or any help, comes back to his house with a bunch of coal, iron and nothing else? :/ If I don't fall into a lava pool or wild mobs slaughter me before I can reach the spawn...

I do.