I didn't really expect the answer to be 100% simple...I have played this game before ^-^
Still, if I can at least rule out any units that say 'advanced' or 'experimental' on their record sheets, that will help. I tend to love Dark Age units, so there is a whole lot I can ignore right off the bat if those keywords mean the unit is going to be verboten for sure. Now, if there are other units without those keywords on the record sheet that are STILL illegal...well, that sucks, but at least it is a, hopefully, smaller list. I did go look through TW as well, so I have a decent feel for what tech is going to be a problem. I may still screw up, but it is much less likely now!
I can't speak for anyone else, but I would be willing to throw out some potentially-OK units if it meant I got a very simple method to figure out if a unit was legal ahead of time. For me personally, having a tag that showed up on the MUL indicating if a unit was legal or not would be the ideal situation, but failing that, maybe a list of approved TRO's or Recordsheet products to pick from? For example, there are perfectly valid units in TRO:3145, but I would be willing to accept not using them if it meant the 'Allowed List' was defined as, "Units that exist in one of these recordsheet products: A, B, C."At the very least, maybe include a URL in the event description to a page with more detailed instructions. Just saying 'Total Warfare' as the game rule set wasn't the easiest thing to parse on my end. I know we are all there to have fun...but we also like winning, so we want to pick units that are good, given the rules of the event.
I don't know that this is a high priority problem, but I know at least one player at the table I was at ended up switching units because what he picked ended up with a hideously bad pilot (he had no idea it would be that way), and the mech he replaced it with...well, it doesn't meet the criteria mentioned in this thread so he probably shouldn't have picked that one either! Also, as I said, I tried to stay with 3050's-level tech (clan...but 3050's clan) to make extra sure I was OK, so I was limiting myself more than necessary because I wasn't sure what was legal. I think that I understnad what to look for now, but it might still be a nice idea to make it easier for other new folks in the future.