how can I tell if I'm in plane with Mars or not?
The number you are looking for is EIn. To get this value low, it's best to launch into the ejection plane using Surface Launch. Set it to "Plan" and it will help you choose the right time to launch with a 90 degree (or as close as possible) heading. These opportunities come about twice per day, so when you are about a day short of TEj, take the next 90 degree launch window, and establish a parking orbit of about 200km.
If you are already in orbit, Orbit Eject also displays EIn. Just under that is a time-to-node (negative values indicate time SINCE last node) counter and a dV (positive is Normal, negative is Anti-Normal.)
EIn should be very close to zero, otherwise there is a plane change included in the ejection burn, which can make it VERY long indeed!
I strongly recommend getting IMFD Full Manual, it includes an Earth - Mars flight that will answer many of your questions.
Proper sequence is Surface Launch (if landed), then Orbit Eject (tune EIn and ejection burn - note that the ejection burn is also the main transfer burn), Then use Target Intercept (after leaving Earth's SOI, set SRC to "x", mode to realtime (not off-axis or prorade)) to make MCC's. Once you are about 2 planetary diameters away from the target (still a bit outside it's SOI) use Planet Approach OR Base Approach to establish a suitable PeA and inclination. Finally, Orbit Insert is used to inset and circularize at the destination.
BTW, ignore the warnings about Orbit Insert - it has been fixed since 5.1 - there is no need to use IMFD 4.2.1 for insertion anymore.
