How to Spot a Bad Tile Job.
A tile job done well and right should last for years, if not longer. Here are some signs to help you indicate that your tile may have not been done properly:

Crooked Tile(s)
Even if your walls aren’t perfectly straight, your tiles definitely should be. Someone who knows what they are doing will use a level, and from there its a straight forward job.
Bad Placement
Most of the time while tiling you want to start from the centre of a wall so you don’t end up with random spaces that need tiny slivers of tile making it look horrible.

Overuse of Grout/Sealant
If someone may be trying to cover up some shoddy work, they may overuse grout to try and cover uneven cuts. This results in the finished product looking cheap and unfinished. A proper tile installer will make the recut on a tile to make the finished product look nice, clean lines and straight edges.

Consistent Grout Lines
Grout lines are meant to be straight, uniform and minimal width. If you use tile spacers you will achieve all of these. The lack of quality and uniform look will show if someone skipped this step and slapped it all together.
Using tile spacers creates uniform, minimal width grout lines.