If you're not used to this, many courses are open.You can look meaningfully at the person about to take the solo. Or you can shout "fiddle solo" Or some more folky interjection like "Take it from then John". (Change this to include their name, so as to be sure of attracting their attention).Sort of pointing your guitar at them can look quite cool too. with blowing a kiss an optional extra.(that might be over-familiar for church use, unless you consider it as the Kiss of Peace).