I take it you have an automatic. I do not, so my advice here is limited.
Many new cars have switched to a sealed or dealer-only check level, where there may be a cap but you have to have a special dipstick that doesn't come attached to the cap. There is a very high likelihood the Sonic is this way.
Otherwise, look for a cap in the engine bay that is low to the trans itself, or a plug somewhere on the trans from underneath that you plug to check the fluid by sticking your finger in.