Using google photos to save all photos in cloud on a mac?

Over the years, i have been having trouble in accommodating my photos collection on the 256GB SSD in my retina Macbook pro. i have tried many different approaches over last few years (exporting albums to external hard drive, writing photos in DVD's, removing duplicates etc.) but nothing ever was enough. so, recently i decided to use Google photos to backup my photos since it gives Unlimited storage options. when i first installed the app, it was showing that i had around 55k photos that were going to be uploaded. i left the process running for 2 straight days and the number came down to 40k. then i had to restart the machine as i have image retention problem in the screen; a problem many mid 2012 retina Macbook pro owners face! after the restart, the countdown came down to 20 and completed in next 2 minutes. now, how can i be sure that all photos did upload? and also, how reliable is this service from google?