Best ways to know your rating in order of reliability:
Play in tournaments (the only real way to know)
Join a club / play many games with someone in person who has an established rating and ask them to estimate your ability (somewhat reliable).
Play a lot of games online where you can view people's profiles. If they note their OTB rating (and aren't lying) then after many games (not a handful) you can see a trend in how well you're doing. (not as good as face to face because in online play people may be distracted or screwing around or may be significantly better or worse in blitz vs OTB). So again this is after more than a few games.
Software is one of the worst ways primarily because a rating is not a measurement like distance or time. It's a statistical measure of performance based on your results. It tries to predict your expect score for future games for that pool of players and is useful or tracking progress and making good pairing / prize distribution. Notice if you lose a game you don't immediately become worse even though your rating will immediately go down. Heck if anything you likely got better for the practice / learn something in the post mortem.
e.g. 2400 on one site is not 2400 on another. 1200 online is not 1200 OTB. 2200 FIDE is not 2200 anything else. And of course a computer can only guess based on how other players happened to answer the same questions. It would need quite a large test sample to be accurate. A book once rated my tactics (I did the test puzzles) at 2300 which was wrong. On one puzzle I was nearly out of time so I guessed a tactical looking move which happened to be a brilliancy from Anand. For that 1 move the author awarded a lot of points (usually he didn't award any unless you saw at least a few moves). In a real game I would never have played it, and if I had, I would have lost :p
tl;dr -- go to a tournament and find out.
Is there a software that can tell you your rating based on the accuracy of your play. Since programs like Fritz can check your blunders and correct your mistakes can't it also give you an approximate rating?