Client converted Traditional IRA to a Roth IRA. 1099R shows -0- taxable & code G. how do I show that the amount converted is taxable ?

Did they bring in the statements so you can see if that's really what happened? 

Ive found that clients don't always know what theyve done for sure. 

I have clients that convert to ROTH every year and theyre not coded with a G.

Sounds like either someone got their facts wrong or their investment person messed up.  Based on the 1099R it sounds like they rolled from traditional to another traditional.  You need to get Lucy in so she can do some splainin.

There is time to make a corrective action for 2023, so it's worth checking on all of this.

https://www.irs.gov/pub/irs-pdf/i1099r.pdf

Code G: "Use Code G for a direct rollover from a qualified plan, a section 403(b) plan, or a governmental section 457(b) plan to an eligible retirement plan (another qualified plan, a section 403(b) plan, a governmental section 457(b) plan, or an IRA). See Direct Rollovers, earlier. Also, use Code G for a direct payment from an IRA to an accepting employer plan, and for IRRs that are direct rollovers."

Which typically has nothing to do with from Trad to Roth IRAs, because those accounts are not under a plan. I would start there, then. Was an employer or employer plan involved? Is the broker with the account working under an employer's plan?

And ask for a Form 5498 for tax year 2023 for this destination account, to see what it was labeled going in. The 1099-R is for the outflow, so it isn't always informational to the taxpayer's action.