how can you tell if a 2015 sentra transmission is slipping in high gear

Hello, you have a CVT - a continuously variable transmission, which means when you have the accelerator engaged aggressively, you'll find that the engine will rev to a high level, maybe 4,000 or 5,000 RPM on the tachometer. This would be normal, as the "power band" of the engine relates to the transmission sweet spot -- which is probably somewhere in that neighborhood of engine speed. As a result, what you are experiencing may be quite normal. But, if you are decelerating and the engine is revving freely, then it could be that there is indeed a problem. A CVT that has poor engagement could need the fluid changed, or the correct fluid installed, if the fluid has been changed recently. There could also be internal damage done to the transaxle -- so it's hard to really know without a trained technician road testing your vehicle with diagnostic equipment on board to see what is happening and how to resolve it. Use this link to find some shops near you that can help with your concern: www.repairpal.com/repair-shops