Unlike Mega Upload achievement, the 1000 Thumbs total can be spread out across multiple workshop uploads. In theory, you can upload up to 1000 uploads and like them yourself, or less if you have enough dedicated friends to help you like each of them.
Progress toward the achievement updates shortly after starting the game and can be seen in notification in the upper-right corner, but can also be checked in console by launching the game with
+achievement_debug 1 launch option and will update periodically shortly after each thumbs is received. The game keeps number of thumbs earned in "NUM_WORKSHOP_THUMBS" value, which can be checked via Steam API request
http://api.steampowered.com/ISteamUserStats/GetUserStatsForG...where "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX" is a Steam API key (get yours at
https://steamcommunity.com/dev/apikey page) and "YYYYYYYYYYYYYYYYY" is your steamID64 (see
https://steamid.io/ or similar).