The only free bitcoin accelerator is
https://www.viabtc.com/tools/txaccelerator/. Other bitcoin accelerators are either not free or they are scam.
If your transaction is not having more than 500 bytes in size and the transaction fee paid is not less than 10 sat/byte, you can use ViaBTC free accelerator.
Others that call themselves free accelerators will keep rebroadcasting your transaction and not accelerating anything.
Is there any way to check if it actually works? I've used ViaBTC accelerator for a bunch of times and it always took hours to get a confirmation even after transaction was already accelerated. So in my case it was kinda impossible to check whether it could take longer to get it confirmed.
It is much better just to change the transaction fee via replace-by-fee instead of relying on accelerators.