Claiming you're low when you are not is going to cause a major headache for the PIC, they're going to have to write that up and they may well be investigated. If it turns out they were lying they would likely find out that that is a career limiting move and if it happens too often then that too should result in consequences. The main reason is that your fake emergency may cause someone else to have a real one.
When you declare a fuel emergency or even urgency, there's often follow up to figure out why (mechanical issue? problem with dispatch? problem with flying technique? exceptional weather condition that could be forecast better? etc). And there is plenty of data in aviation to know what happened.
Dispatch knows how much fuel they say they put in.
Your flight time, speeds, and profile are known.
ACARS may be reporting fuel use throughout the flight.
etc, etc, etc.
Random spot checks. Every day at every airport some of these will get verified. Also, the next pilot would have to be willing to cover for you because they are going to have to falsify their records to make your trick invisible. You record the amount of fuel in the tank when you take command of the aircraft, the amount of fuel that was loaded and from that it is trivial to compute how much was left the last time it landed.
Lets say a plane crew claims low fuel.
The pilot in charge has to file a writeup.
When someone accepts the writeup, there's a random chance it's selected for followup. If/when they discover there was enough fuel, it will affect the career(s) of person(s) involved.
First, generally, people don't like having to do paperwork, and especially don't like doing paperwork to help you land a little quicker.
While one time may not be a fireable offense, you will find you career affected in the number of ways people can find to be uncooperative with you, or not support you when you attempt to advance your career within the company.
Developing a habit would lead your interlocutors to escalate the situation, which would lead to discipline up to and including the company firing person(s) involved.
What’s the mechanism for them to get caught?