Without any knowledge of their code, it is very difficult to find out how their randomness is generated. It would take knowledge in cryptography and most likely a very large number of hands.
A method that anyone can use, however, is tracking it manually and looking for patterns. You can do this most easily by making a program that will gather the hand information. Worst case scenario would be manually logging all of the cards dealt over as many hands as you can and analyzing the data using statistics depending on what your concerns are.
In my opinion, this is a waste of time. Especially in the US, these mobile apps are not regulated and they have no incentive to make sure their randomness generators are truly random. If these apps use real money, they would have an incentive to not have true randomness because it will make them money if they run a shady business. Apps like the WSOP app that use play money have an incentive to make rare hands more likely because it generates excitement and action that players like to see.
No point in taking a lot of time to debunk these apps. Even if they are caught red handed, they will barely be affected. Their business will face no legal action and they will still find just as many suckers to deposit and play on their app. It is a better use of your time to find an app that you trust with a large player base, preferably regulated and legal with a good reputation.