A properly done conversion should have included a label affixed that states clearly R134A and the date it was performed as well as special threaded shrader valves with blue caps.
Besides that the contents can only be verified by a test using the available testers, when an AC service shop does any work they have to (by law) recover the gas into appropriate tanks with different threaded hoses for each system.
Here's the racket in Calif anyhow but it may not be the same elsewhere:
The firms that buys recovered gas will always check it with a tester before accepting it and if the R134A tank for example has any traces of a hydrocarbon (flammable) gas or other than 134A they won't buy it back. The shop is stuck with paying to have it removed, instead of receiving a payment or being allowed to reuse it.
Without any records and if it doesn't clearly have the sticker and the blue caps on the fittings then you need to have it tested at an AC shop, I doubt they would do it for free but it shouldn't be very much. Otherwise you are at risk of messing up the system in a big way. And no telling what kind of oil is in it!

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body. '87 300D w/ Sportline suspension. No more '79 300TD w/ 617.952 engine at 367,750, it was my annual driver to Texas and I sure don't miss it poor A/C heater system!