If you are going to lean on an engineering firm to reach bigger work, the first question is not about price, portfolio or turnaround. It is whether they also do electrical contracting.
That one fact decides whether you can open a job up to them, or whether you will be managing a conflict of interest for as long as you work together.
What to confirm, in order of weight
1. Does the firm do electrical contracting?
What it is. A firm that only does engineering has no way to bid against you. A firm with a contracting arm always has the option, whatever it does with it in practice.
The trade-off. Plenty of firms will give you the answer you want to hear, so check it rather than take it. Do they self-perform installation work? Are they licensed as an electrical contractor anywhere you both work? On past jobs, do they show up as the contractor or only as the engineer of record? That last one is the hardest to talk around.
2. Is there a licensed PE who will stamp the work?
What it is. The credential that lets you bid the work at all.
The trade-off. Easy to verify through the state licensing board, which is why it gets checked first and leaned on too hard. A seal tells you nothing about the conflict question or about how a firm behaves once it is on a job.
3. Do their engineers visit the site?
What it is. Whether the engineering gets done from drawings or from the actual conditions in the building.
The trade-off. Site visits cost hours and they catch what the drawings don't show. Existing conductor sizes, what is actually feeding what, equipment that got replaced without anybody redlining the one-line. A firm working only from documents is cheaper on the proposal and dearer on the project.
4. Do they stand behind the work when something goes wrong?
What it is. What happens at two in the morning when a setting is wrong and a client is waiting.
The trade-off. The hardest one to verify in advance and the easiest to check with references. Ask about a job that went badly rather than the ones that went well.
Why that first question comes first
The value in a partnership like this comes from depth, and depth needs you to share openly. The firm does its best work with the RFP, the client, the site and the details in front of it. Hold things back to protect yourself and you get worse engineering and a thinner relationship for your trouble.
A responsive, credentialed, accountable firm that also bids construction can still bid against you. The other three questions are worth asking once that one is settled, and not much before.
With the conflict off the table you can chase larger, engineering-required work without wondering whether your partner will appear on your client's next project as a competitor. That is what makes it worth settling on one firm instead of shopping every job, and settling on one is what lets the relationship build.