First of all there are two overlays. One in DCS for SRS (LCtrl+LShift+Esc) and SRS overlay (you can set key bound set in SRS client). DCS SRS overlay give good info on, if you are connected at all, what freqs you are on, how many people are on that freq (if Server allows it, most does), what radio you selected as active one, and if your Push to talk button works (of course mic test requires you actually talking and hearing response). SRS overlay allows you to set freqs for planes which lack clickable cockpits (FC3 planes for example) or sometimes (depending on server) admin gives extra radios to planes which historically had less of them (eg. Blueflag Cold war). Using SRS overlay is only way for FC3 planes to set radios so you have to bind that keys in SRS client for that. Mouse cursor in FC3 planes still exist but as SRS client overlay is technically different app Windows only renders mouse cursor when its on SRS overlay itself. You can also active mouse cursor in FC3 planes (i think its Lalt+C by default).
Comms check is actual procedure, and almost everybody does that every time anyway, so noone will be angry at you by doing that. Also you can check if u are connected in SRS client itself as well.