It is a shame reddit lets people hide their post and comment history now so there can be no identifying signals about astro-turfing or bots. I'm sure this is ostensibly about preventing harassment, and in actuality about disguising bot behavior driving engagement. Or maybe I'm just extra cynical this morning.
Just go to the profile in question and search their profile with an empty query; that'll show all the hidden comments and posts.
Edit: at least on the web variant, it looks like they finally blocked that ability.
IIRC you just have to use an asterisk or something now, and I think the search is broken on OldReddit…