I guess my first question should be, does my ISP know when I'm using Tor? I read somewhere that they do. In that case, the anonymity offered by Tor is rather limited. If someone suspects I posted something naughty, they only need to check with my ISP and match the time of posting with times I was using Tor. (unless I am willing to use Tor all the time, which I am not)
There is no long term credible way to deny the ISP provider from figuring out you are using Tor. You may use bridges or pluggable transport bridges to temporarily deny ISP from knowing that you are using Tor, but if any credible investigation arises regarding this matter, your ISP and bunch of other people would know that you were using Tor.
Yes, your ISP knows you use Tor. You can use Tor bridges to circumvent this.
Yes, an ISP can assume or predict you are using Tor if they are monitorring your traffic, but they can't know what hidden services you're visiting, nor the content you are sending or receiving from the Tor network. It can be detected from the IP addresses of Tor relay nodes you're connecting through.
But recently, ISPs got smarter and started to use Deep Packet Inspection (DPI) to recognize Tor traffic even when you're connecting to IP addresses that are not on their list of Tor relay nodes or bridges. Lucklily, there is a countermeasure for that, which is using pluggable transports. Pluggable transports make the traffic between you and Tor bridges look innocent and much harder to detect. See Pluggable Transports page for more answers.
If all fail, you can try to connect to Tor from a public WiFi network, or using a VPN connection.
TOR has an option named OBFS (obfuscator) and it is designed to bypass possible filtering by ISPs in accessing TOR. Actually it an option from pluggable transport.
I've put a screenshot of where to find it. Firstly click cancel when TOR is starting up, then click reconfigure, then check TOR is censored in my country.