I got the information here
One option to check RAM usage is to enable Developer Options.
So, just follow the steps and get your thing done. Just go to Settings > About Phone and tap the Build Number repeatedly until the notification pops up and says “you’re just 1 click away to become the developer,” or if it pops up “you’re now a developer,” this means you have successfully done the first step.
Now, go back to Settings where you’ll find a new option “Developer Options.” If you’re using an older version of Android, then you may see “Process Stats” option that will tell you the average and maximum RAM usage. This option also shows the list of currently running processes.
But, if you’re like me, running Android Oreo (or maybe Nougat), then you may not find the Process Stats option. This may be available as “Running Services” under the Developer Options.
Clicking a particular app will open a new window just as we saw in the first method in older versions of Android. Here, you’ll be able to check which app is eating how much RAM.
Also, the same site suggest a third party app. I didn't test it, but it seems to be good according to the reviewers...
However, if you are looking for strictly benchmark, these are some options:
I'm not completely sure how acurate these apps are, since it is pretty hard to be sure. I would look into the user's comments to choose the best one (I've found other apps also, but checking the comments made me believe they were fake benchmarks).
I hope it helps you...