Assuming you have an automatic transmission, the process for checking your transmission fluid when hot is as follows:
1. Park your vehicle on a level surface and set the parking brake.
2. With the engine idling, pull out the dipstick and wipe it clean with a lint-free cloth or paper towel.
3. Insert the dipstick all the way back into its housing, then remove it again to check the fluid level. The fluid should be at or near the full line on the dipstick. If it's not, add more transmission fluid until it reaches that level.