It is not possible to RNG abuse your TID or SID in Sword/Shield. This tutorial is only for finding your internal TID/SID on an existing save file. There are three possible methods you can use, these are:

Using SysBot.NET

SysBot.NET is a bot service hosted by members of the community, typically found in Discord servers. If you do not have access to a SysBot, you can find a public server with a quick Google search.

SysBots come with dump functionality, which dumps the data of the shown Pokémon into a .pk8 file, this file is then shared into your Discord messages with the bot.

The file can then be manually inspected using a program like PKHeX to view your TID/SID combination. This method is recommended as it takes the least effort, but does require a Nintendo Switch Online subscription.

Help from a friend

If you have a friend with a CFW Switch console, they are capable of dumping the file for you, but they will need access to PKHeX to do so.

You can also have a friend check your TID/SID combination if you do not have access to PKHeX. PKHeX only works on Windows devices. This method is only recommended if you do not have any access to a Sword/Shield SysBot.

Finding a shiny Pokémon without RNG abuse

If you do not have access to the above options at all, you will need to manually search for a shiny Pokémon in the wild. It is possible to trace back to the frame that generated the shiny using SWSH OWRNG GENERATOR GUI.

This method is the most time consuming and luck dependent, and requires you to leave the game open immediately after capturing a shiny Pokémon.

Requirements:

• PKHeX
• Nintendo Switch Online Subscription
• A Discord account (If using SysBot.NET)

Tutorial:

• Extract the contents of the PKHeX download into an empty folder in a known location on your PC.
• Click Options at the top of the window, then click Settings, change the Blank Save Version to Sword or Shield.
• Dump the .pk8 file of a Pokémon that was caught in the save file you wish to check the TID/SID on with a SysBot or with a friends help.
• Download the file and drag it into the PKHeX window, you should now see the details on screen. Head over to the OT/Misc tab to view Trainer Information.
• Hover over the TID field with your mouse, here you can see the 5 digit TID/SID combination. Take note of this as you will need it every time you want to RNG abuse shiny Pokémon.

Requirements:

• Finding a random shiny Pokémon via an overworld encounter
• Keeping the game open (but paused) immediately after capturing a shiny

Tutorial:

• Run around in the overworld and encounter Pokémon until a shiny Pokémon appears.
• Capture the Pokémon, then quickly pause the game after doing so in order to pause the RNG state.
• Identify your current seed using the Retail Seed Finder window, then click the Search for TID/SID checkbox on the main window.
• Enter your search range, this should be however many advancements you believe has occurred since you encountered the Pokémon (If the above steps were closely followed, this should be a few thousand advancements at maximum).
• Hit search, and find the frame that generated your shiny Pokémon. You should now also see the TID/SID combination that made the Pokémon shiny.
• Take note of these values, you will need them in order to hunt shiny Pokémon, and to avoid repeating this process every time.