I would charge them both then check. All sorts of meters avaialble for checking. Cheaper than two new batteries and a good thing to have in your tool box.
I put a little "Hobbico Voltwatch" or the "MPI" lighted charge jack/switch on all of my planes. Both have lights to indicate battery voltage. MPI can handle 6v.
Reassuring to simply look and see if your RX battery is OK. Reminds me its on if I forget too.
I have had 3 transmitters and all had a guage or a digital reading for voltage.
If yours Tx does not have a meter get a battery checker of some type. Futaba makes a $40.00 BR-2000...j-type receptacle or you can get the CDR-5000 for $450.00. Hitec makes a Power Mate 2...BEC connector for $29.00 to name a few. With the electric craze going on there is no limit to what you can get. Just make sure you get the correct receptacle for your brand of electronics.
I personally let a ni-cad sit for two years and it is still going strong after 2 more years of using it often.
Just check it. You sure do not want your plane to crash because of that. I will not fly it until I know the batteries are strong in my planes and the TX.
New batteries are not that much either. For me a discharger/charger/checker/etc is a must. I recently joined the craze.
Happy flying
Michael