There is absolutely no way you can be assured of anything other than that there is a 100% chance that you may have to pay the over-size fee.

You can check in lots of things. But, if they are oversize, you pay the fee if the agent seeks to collect it. There will be a 100 posts here, some from people who caught a break and others from those who didn't. None of them prove anything other than that agents are people.

You have two choices:

1. Use a bag which meets the linear maximum; or
2. Use your current bag and if you are told that you must pay, pay up.