Assumed Area of Steel In Existing Concrete Beam

You could assume the minimum flexural steel for that era (probably rho=.0033) and go from there, but this sounds like a dangerous approach. I'd make every effort to find the original drawings and if that fails, do some non-destructive investigation, like x-rays or chipping out the concrete and visually counting the bars.
I wouldn't want to stake my professional career on an assumption that they used minimum reinforcing.

My additional tributary loading would be approx. 60% to one side of the beam, I'm going to retrofit another beam next to the existing beam to take the new load. That I can control instead of taking a risk as you stated.

Thanks for the input.

They have some excellent cover meters that can help determine the bar sizes and stirrup spacing. As a caution, some beams were reinforced using 'trussed' type bars that bent upwards from bottom reinforcing to become top reinforcing contributing to the shear strength. Typically the reinforcing may have had a yield less than 40 ksi, 40 ksi, 50 ksi, or in rare occasions 60 ksi and stirrups may have a lower yield than the main reinforcing. If there is an area you can get a coupon to test, this would provide the best information, else, as noted above, you have to rely on NDT.

For the longest time, I kept a metal tag with the reinforcing mark number on it... I found this in an old reinforced concrete beam that had been constructed about 1920. The rebar was twisted for strain hardening and bond... and all the steel was bundled together complete with the metal tag. There were four bars neatly tied together as one; don't be too surprised about what you find.

Dik

Retrofitting a new beam is definitely the way to go.

BA

Jedclampett has it right - assuming rebar reinforcement can get you in trouble. My experience is that rebar meters do not provide good data. Best bet is to chip away and see if you can find some rebar or design something that works regardless of what the conc beam has to offer.