Determine ELO strength without playing in FIDE tournaments I don't play club chess or tournaments, how can I determine my ELO strength? 
I often play on chess.com how well does it compare to the FIDE system? (I am aware that one cannot account for people cheating online)
This is going to sound tautological, but Elo is inherently a measure of how you do in tournaments. Your question is sort of like asking "I don't run, how can I determine how fast I can run a mile?" We could guess from how generally fit you look, but in reality you just have to go run a mile to find out.

There are correlations between various online ratings and a FIDE rating, but they're just correlations; the context of how you play online is very different from playing live (for one thing, online games are usually much faster).

I can tell you that the first time you play in a live tournament you will likely perform 100-200 points worse than you might expect from such a correlation, just because you're not used to the environment of a face-to-face tournament (moving real pieces, using a clock, writing down your moves, having your opponent right there in front of you, being distracted by other games). Luckily it does not take long to settle in.